Posts Tagged ‘twist rate’
by Tom Gaylord, a.k.a. B.B. Pelletier
I thought I’d provide some thoughts on how this blog has helped me become a better shooter. We just finished the Twist-rate test last week, and I wanted to share with you some things from that test that have impacted my shooting.
I’ve been using the output of the test to guide my own shooting with both firearms and airguns. I did test the swaged bullets in my muzzleloader already, but I discovered that there’s a lot more to learn. I can’t get the bullets I’ve swaged to group at 50 yards to save my life! I get maybe 1 out of 3 shots to land on paper. I think the problem is that I’m driving them too fast. I’ve been using the same load of black powder that I use for a patched ball, which seems like double the amount needed to get good results. When these bullets are fired in airguns, they’re shot at between 400 and 650 f.p.s., and I’ve been launching them at 1,000 to 1,200 f.p.s. I think I’m blowing them apart!
I plan to load less than half the amount of black powder to see if I can get the velocity down around 650-800 f.p.s. The twist-rate test told me these short bullets don’t need to be spun up as fast as I’m shooting them. At least, I think that’s the case. More later when I actually have something to show you.
A friend talked me into buying some Accurate Arms 2230 powder for my AR-15, but when I tried it with the 69-grain and 77-grain bullets I normally shoot, it was lousy. The burn rate of this new powder is way faster than the Varget and Reloader 15 powders I’ve been using, and both given such good results. I figured the problem was that I was trying to push a bullet that was too heavy with a powder that was too fast.
I still can’t get over how accurate my AR-15 is with the right loads.
Ten shots into 0.54 inches at 100 yards is pretty good for an AR-15.
I loaded some lighter 55-grain bullets with the lowest charge of 2230 the book recommends. But the results were poor, again. Oh, they were twice as tight as before, when I shot the 77-grain bullets; but with 10 shots going into 1.808 inches, they still aren’t worth the trouble. It may be that my 1:8″ twist is just too fast for lightweight bullets, but I still need to experiment some more.
Ten rounds into 1.808 inches at 100 yards is terrible for a rifle that can do so much better.
I updated you on the Ballard situation a few weeks ago. I told you that I discovered the Ballard’s rifling has a twist rate of 1:20″, while Winchester rifles of the same caliber (.38/55) have twist rates of 1:18″. Given the slow speeds I shoot, the difference is significant. Bullet molds and cast bullets that are available today are all made for the Winchester, which is far more common than a vintage 1886 Ballard.
My vintage Ballard turns out to have a twist rate that’s on the slow side for the bullets that are available.
What this means is that I’ve been trying to shoot with bullets that are too long and heavy for the twist rate of my rifle! It took the results of the twist-rate test to drive this fact home. The solution is not as straightforward as it sounds, however. You might think I could just have a custom mold made for a shorter bullet. That’s possible, but I first need to think some things through. For example, would the rifle do better with a full charge of black powder and the bullets that I already have?
I got very discouraged when I learned that my rifle had a slow twist rate. I even considered selling it and buying a rifle that was more suited to what I want to do — which is shoot 10 shots into less than one inch at 100 yards with non-optical sights. But when I tried to list the rifle for sale, I found I could not let it go!
No matter what I do, I just cannot seem to get the Ballard to group much better than this at 100 yards. Ten went into 2.654 inches.
So it’s back to the beginning with the Ballard. I have to figure out how to make this old beauty shoot in its current form because it is too valuable to modify. And everything I do from this point forward will be driven by the slow twist rate!
The next twist-rate test
We’ve finished the initial twist-rate test, but we aren’t finished with the special barrels. Several of you have asked me to test the 1:12″ barrel with heavy pellets that are longer. I’ve decided to test it at 50 yards with the heaviest, longest pellets I can find. But besides that, I’ll also test the JSB Exact Jumbo RS pellet in the 1:22″ barrel at 50 yards. The RS is 0.002″ longer but nearly a full grain lighter than the Premiers that were used in the first test, so we may see an accuracy improvement over the 2+ inch groups the Premiers gave. Being lighter, the RS will travel a little faster, which means it will also spin faster, and that may help the accuracy. We won’t know until we try it.
This is good
I told you when I started the twist-rate test that we might find specific things to test in the future. These are those things, plus a few more. What a ripple effect this has had on all of my shooting!
by Tom Gaylord, a.k.a. B.B. Pelletier
This is the summary report in this series. I’ll give you my thoughts on how this test went, and I expect you to comment, as well.
Three barrels were used in this test. One was the factory barrel that comes with the .22-caliber AirForce Talon SS. It’s a 12-inch Lothar Walther barrel that has a choke of about a half-thousandth inch reduction in the bore diameter for the final 2 inches of length. That makes all the pellets of uniform size as they leave the muzzle, and it may potentially stop any in-bore wobble. This barrel has the standard airgun twist rate of 1-turn-in-16-inches of bore travel, written as 1:16″.
The other 2 barrels were made by Dennis Quackenbush. Neither barrel is choked. One is a 1:12″ twist; the other is a 1:22″ twist. They’re also about 12-inches long and are held in the gun by AirForce Talon SS barrel bushings. Several comments have suggested that because these barrels are different than the Lothar Walther barrel, this test is somehow not fair. But the results of all the shooting prove otherwise. Sure, there are variations from barrel to barrel, depending on the power used and which pellet was shot. But the results are so close between all 3 barrels that whatever differences there might be are overridden by the similarities. In other words, I’m suggesting that if Lothar Walther had made all 3 barrels, there would be similar differences.
The 3 barrels used in the test. Factory barrel in the middle.
I believe the twist rates are what drive the results. We weren’t searching for the most accurate barrel in this test. We were looking for behavior changes as conditions were changed. And we got that.
The first thing that was tested was velocity. Both pellets — the 14.3-grain Crosman Premiers and the 15.9-grain JSB Exact Jumbo were shot from all 3 barrels at each of 3 predetermined power settings. These settings were marked on the gun so they were kept constant throughout the test.
The power settings were the power indicator screw all the way to the left (the lowest setting), and the power screw centered on each mark (settings 6 and 10).
I reviewed the velocity for you in Part 8. Here’s a summary of that report.
In all cases, the velocity increased the most between power setting zero and setting 6. The velocity increase from setting 6 to setting 10 was always smaller than the increase from setting zero to setting 6, and that’s irrespective of the twist rate or which pellet was shot.
What you’re seeing here is the slowing down of the rate of velocity increase as the air flow increases. That’ll become clear in a moment when I discuss the rifle’s maximum velocity potential.
As the twist rate slowed (1:22″ is slower than 1:12″), the velocity increased at most power settings with most pellets. There was one instance with the 1:22″ barrel when the JSB Exact pellet actually went 2 f.p.s. slower at setting 10 than at setting 6; but with all other barrels and pellets, there was always a velocity increase as the power setting went higher.
Focusing on the 1:22″ barrel for a moment, we see that the velocity increases between setting 6 and setting 10 were not as great as they were in either the factory (1:16″) barrel or the 1:12″ barrel. This suggests what we have suspected all along — that the twist rate of the barrel does slow down the pellet as it gets tighter. And we can see from this test that the phenomenon is most apparent at the lower power settings. At the higher power settings, the differences seem to shrink, indicating that the influence of the power setting is overriding the influence of the twist rate. I believe this is an important finding, and it sets up the next observation, which is that the top velocity of the gun was fairly close for all 3 barrels, regardless of the twist rate. The type of pellet made more difference to the top velocity than the barrel twist rate did.
It should be obvious from these results that the Talon SS powerplant has upper limits that cannot be exceeded by forcing more compressed air through the barrel. This illustrates the relationship between barrel length and velocity in a pneumatic airgun.
A second thing I found interesting is that power setting 6 is very close in performance to power setting 10. In the case of the 1:22″ twist barrel, it’s remarkably close…but it’s close for all three barrels. A prudent airgunner might consider this when setting the power wheel adjustment on his Talon SS, knowing that a lower setting uses less air, yet gives velocity that isn’t that much slower.
A third thing is that the velocity performance of the 1:22″ barrel is so good at power setting 6 that it makes power setting 10 useless. Take that thought just a little farther, and you’ll see that all power settings above setting 10 are pretty much a waste of air in a Talon SS with a 12-inch barrel, regardless of which pellet you use.
Next, I tested all 3 barrels with both pellets shot at all 3 power levels at 10 meters (11 yards) and 25 yards. Following that, I tested all 3 barrels and both pellets, again, at 50 yards, only I didn’t use the zero power setting. This was where my eyes were opened regarding the effects of twist rate.
I analyzed the accuracy in 2 different reports. One (Part 9) was the 10-meter and 25-yard accuracy and the other (Part 12) was the 50-yard accuracy, alone. Now, with the table above we can combine these results and analyze all the accuracy data together.
The first observation I’ll make is that at 10 meters, I got 10-shot groups that ranged from as small as 0.092 inches to as large as 0.578 inches. The factory barrel gave the best results with the JSB pellet; but with the Premier, it was the 1:22-inch twist that did the best. Curiously, that pellet and twist rate didn’t seem to change that much as the power was increased (at 10 meters). With all other barrels and pellets, the group size did change a lot as the power changed.
It’s too simple to say the factory barrel with the 1:16-inch twist rate is the best; but of the 3 twist rates in this test, it certainly is the most flexible across the board. However, you’ll notice that the 1:12-inch twist barrel did shoot the best single group (with JSB Exact pellets) at 50 yards. That group is so close to the Crosman Premier group shot by the 1:16-inch barrel that I can’t call a clear winner — BUT — here is what I CAN say. The Quackenbush 1:12-inch twist barrel is certainly capable of shooting 50-yard groups at least as tight as those shot by the Lother Walther barrel; and in my mind, that puts the barrel-equivalency question to rest.
Another observation is that the 1:22-inch twist barrel was just as good at 10 meters as the other 2 barrels, in general, but look at how the groups opened at 50 yards! That says something very strong about the relationship of the twist rate to accuracy. And it also brings up a second observation.
Premier pellets and JSB pellets performed differently throughout this test. Just look at the 50-yard results for Premiers and JSBs with the 1:12-inch twist barrel, and you’ll see what I mean. This is one more bit of evidence that barrels have preferences for certain pellets.
This will be my final remark in this series of reports, and it does not come from the data collected in this test but from the 5-part test of the Diana model 25 smoothbore. In that test, we saw that the smoothbore was able to place 10 JSB Exact RS pellets into a group measuring 0.337 inches at 10 meters. However, at 25 yards, the same pellet loaded the same way made a group that measured 3.168 inches. That difference tells us clearly that spin and not aerodynamic drag is the main key to pellet accuracy. I think we now see that twist rates do matter a lot, and the standard rate is the best all-around rate for now.
At 10 meters, 10 JSB Exact RS pellets made this 0.337-inch group.
At 25 yards, the same JWB-Exact-RS pellets, seated to the same depth, made this 3.168-inch group. They are clearly not accurate after 10 meters.
by Tom Gaylord, a.k.a. B.B. Pelletier
Today, I’ll report on the final test in this series. This isn’t the final report — just the final test, which is the barrel with the 1:12″ twist, shooting at 50 yards. Get ready to be surprised. I know I was!
I did this test on the same perfect day as the factory barrel that was reported last week in Part 10, and the weather was perfect most of the time. From time to time, there was a very slight breeze that I waited out before shooting. The shooting conditions were as good as they get.
I used the same two pellets we’ve been shooting all along, and they were shot at power settings 6 and 10…just like the other 2 barrels that went before. The gun was shot while resting on a sandbag that’s very stable. When the tank was filled or the power was changed, I always shot one shot to settle the valve. Experience tells me that’s all that’s needed.
The tank was filled to 3,000 psi.
Power setting 10
I first shot both pellets on power setting 10. And 14.3-grain Crosman Premiers were hitting low and to the left. One of them only nicked the target paper, so I photographed the target before taking it off the backer paper, so you could see the complete group.
Here are the two 10-shot groups of Premiers. Notice that they’re hitting to the right of the aim point, which is the center of the bull they touch. The group shot on power setting 10 is at the top, and it shows why I like to use backer paper when shooting at 50 yards.
Then it was time to test the 15.9-grain JSB Exact Jumbo 15.9-grain pellet on power setting 10. This is where the surprise comes! Ten pellets made a 1.259-inch group! If you check back with the results the factory barrel gave, you’ll see that this group is very close to the best group made by the factory barrel (1.153″ for 10 shots with the same JSB pellet on power setting 6), and it’s equal to the group that was shot on power setting 10 (1.283″). This addresses a question many of have had from the beginning of this test — namely, are the Quackenbush barrels equal to the Lothar Walther barrel?
With so little data, it’s impossible to say if these two barrels are exactly as good, but what we now can say is that one of the Quackenbush barrels gave some groups that are at least equivalent to those from the factory barrel. The difference is so small that it might be due to the twist rate rather than the quality of the barrels. That was the position I took at the start of the test, and I think this demonstrates it was valid.
Power setting 6
Next, it was time to test the 1:12″ barrel on power setting 6. Ten Premiers went into 2.234 inches, which is only slightly smaller than the same pellet on power setting 10. As before, the pellet stuck the target low and to the right.
It’s official — the 1:12″ barrel does not like Crosman Premiers out at 50 yards. But that wasn’t the only pellet in this test.
With JSB Exact Jumbo 15.9-grain domes, the barrel did nearly as good on power setting 6 as it did on setting 10. Ten pellets made a 1.363-inch group. Like the Premiers, the JSBs also performed about the same on setting 6 as on setting 10. But that’s not the real lesson. The real thing to note is that the 1:12″ twist barrel was not as good at 25 yards as the factory barrel, yet at 50 yards it almost caught up. In other words, the accuracy of the factory barrel degraded faster as the distance increased than the barrel with the faster twist.
What has been learned?
This is not the final report. I’ll add these results to the previous summary report given in Part 9, and we’ll be able to see all 3 barrels at all 3 distances with both pellets at all 3 power levels. But if I had to give a quick analysis, I’d say the 1:12″ twist barrel surprised me at 50 yards by being better than I expected. At least it was better with the JSB pellets.
And that fact alone — that a barrel can be so much better with one pellet than with another — is good to know. This test has demonstrated that principle beyond all doubt.
A lot more testing needs to be done to thoroughly see all the relationships, but I’ll tell you what I know in the next report, which will be the final report for this series. I think we can advance our knowledge of how pellets perform by quite a bit by combining the results of this lengthy test, the smoothbore test and the pellet velocity versus accuracy test. We’ve been exploring this theme for nearly 2 full years now, and I think we’ve learned a lot!
by Tom Gaylord, a.k.a. B.B. Pelletier
Continuing our look at the 3 different twist-rates, today I’ll shoot the factory Lothar Walther barrel at 50 yards. The factory barrel has a 1:16″ twist rate that has become ubiquitous for airguns and is the very thing this test is designed to examine. Last time we looked at how the 1:22″ twist barrel did at 50 yards, with 2 different pellets fired at power levels 6 and 10. Today, we’ll see the same thing with the factory barrel.
This test was performed yesterday, and the range conditions were perfect. There wasn’t a breath of air to be felt for most of the shooting session, and only an occasional puff of air later on in the morning after I swapped barrels for the final test. I’ll report on that set of results in the next report. Today is devoted to the factory barrel.
The AirForce Talon SS shoots with a fill of 3,000 psi, so before the test I filled the reservoir. Twenty shots would be fired at power setting 10 and another 20 at power setting 6, plus one shot at the start of the test and when the power wheel was changed. I haven’t reported that fact until now, but it’s my standard practice when shooting at 50 yards with a Talon SS.
Valve needs to be exercised
I have learned that the Talon SS valve needs to be fired one time following power adjustments to get it set at the new power level. The first shot will usually be like the gun was on the previous power setting, but the second shot will be solidly at the new setting; so I always take one shot to set the valve with every fill and at every power change.
Shooting off the bench at 50 yards on a perfect day with the Talon SS was enjoyable.
Power setting 10
I started with power setting 10; so when it was time to shoot on power setting 6, the reservoir would have less than the full fill pressure. That way, I knew the gun would be right in the middle of the power curve.
The first pellets I tested were 14.3-grain Crosman Premiers. Because the groups had dropped so far below the aim point in the previous test with the 1:22″ barrel, I dialed up the elevation several clicks for this test. I was hoping to hit the bullseye with the new sight setting, but Premiers on power setting 10 still dropped about 3.50 inches at 50 yards. Ten Crosman Premiers went into a group measuring 1.567 inches between centers. While that’s on the large side for a Talon SS in my experience, it was still a very round group.
At 50 yards, 10 Crosman Premiers went into this round group, which measures 1.567 inches between centers when the power was set to 10.
JSB Exact Jumbo
Next, I switched pellets to 15.9-grain JSB Exact Jumbo and shot another group of 10. This was also at power setting 10, so the gun’s valve did not need to be awakened. These pellets hit about 2.50 inches below the aim point, so they landed higher than Premiers at the same sight setting. This time, the 10 pellets went into 1.283 inches — a much better group than the Premiers, though there was a hint of vertical elongation to this group.
Ten JSB Exact Jumbo pellets made this 1.283-inch group at 50 yards on power setting 10. This group is slightly vertical.
The difference between the Premier group and the JSB group was evident through the scope without walking down to the target. Clearly this barrel likes the JSB pellet better. What did surprise me was that even on this perfect day I did not shoot a group smaller than one inch. I’ve done that so many times in the past that I sort of expected it — especially on such a perfect day. Well, it just demonstrates the difficulty of shooting such tight 10-shot groups at this distance.
Now, I cranked the power wheel down to 6 and shot both pellets, again. The first shot settled the valve at this new setting, then the groups began in earnest. JSBs went first since I’d just finished shooting them at power setting 10.
This time, 10 JSB pellets went into a group that measured 1.53 inches between centers. It was larger than the same pellets on power setting 10, but smaller than the Premiers on setting 10. The group is fairly round, though most of the shots are clustered on the right side.
Ten JSB Exact Jumbo pellets made this 1.53-inch group at 50 yards on power setting 6. Notice that 7 of the 10 pellets landed on the right side of the group.
The final group was 10 Premiers fired on power setting 6. They made a group measuring 1.261 inches, which is the best group of the 4 shot with this barrel. It’s more open than the best group of JSB pellets, but the overall measurement places it at the top of the ladder.
Did you notice that the pellets crossed in their performance relative to power, with JSBs doing best on power setting 10, while Premiers did best on power setting 6? I wouldn’t make too much of that because we don’t have enough data to make any conclusions, but it is interesting. In the macro, it does appear that Premiers prefer lower power while JSBs prefer higher power — at least at 50 yards.
I have to admit that I was surprised not to see even one group that measured less than one inch. I’ve shot so many small groups with this rifle that I expected it this time, at least with one of the four groups that were produced.
In the next report, which is the same shooting done with the 1:12″ twist, there will be a surprising result. So, there’s still more to come.
by Tom Gaylord, a.k.a. B.B. Pelletier
Today, we’ll begin looking at the effects of the rifling twist rate on the accuracy of our test AirForce Talon SS rifle in .22 caliber at 50 yards. If you’re prone to jumping to conclusions before all the data is in, I have to caution you that today’s test will look bad because I’m testing the custom barrel that has the 1:22″ rifling twist. We know from the earlier tests that this barrel was most accurate at 10 meters on power levels zero and 6. Above that power level and also out at 25 yards, the accuracy of this twist rate broke down. So, it would be reasonable to assume that this barrel will give results that are even worse at 50 yards.
That didn’t stop me from trying my hardest to shoot well. I was able to watch each pellet go into the target paper because of the distance, and that was disconcerting when the pellets landed so far from the aim point and from each other. Let’s take a look at how the rifle did.
The day was nearly perfect, as it has to be to get good accuracy from pellets at 50 yards. The air was calm, except for some light breezes from time to time. I was able to work around these breezes and get the results I was after.
I decided not to test the rifle on zero power because of the long distance to the target. Any breeze would have so much time to blow the pellets off course that I felt it wouldn’t prove anything. So, both pellets were shot on power levels 6 and 10. That’s how I’ll test all 3 barrels.
You may remember that this barrel produced velocities that were very close to each other at power levels 6 and 10. With 14.3-grain Crosman Premiers, the respective velocities were 840/854 f.p.s.; and with 15.9-grain JSB Exact Jumbos, the velocities were 817/815 f.p.s. We expect the pellets in this test to go to the same place on the target, and I would expect the two groups for each pellet to be pretty similar in size.
I started with Crosman Premiers and the power set to 10. I did not adjust the scope since completing the 25-yard accuracy test and the center of the group landed about 3.9 inches below the aim point. Ten pellets went into a group that measures 2.04 inches between centers.
Ten Crosman Premiers went into 2.04 inches at 50 yards on power setting 10. The center of this group was about 3.90 inches below the aim point with the scope set for 25 yards. The pellet at the top center is part of another group — not this one. I did account for the full size of the pellet on the left that just clipped the edge of the target paper.
On power setting 6, the center of the group also struck the target 3.90 inches below the aim point. These measurements of the groups are just approximate since the center of each group was difficult to locate precisely. The 10-shot group size on power setting 6 was 2.607 inches between centers. This is slightly larger than the group shot on power setting 10.
JSB Exact Jumbo
Next, it was time to test the JSB Exact Jumbo pellet. I started with power setting 10. The center of the group landed about 4.25 inches below the aim point.
Ten pellets shot on power setting 10 went into a group that measures 2.509 inches between centers. The group is much taller than it is wide.
On power setting 6, the 10-shot group size was 3.222 inches between centers. This group is considerably wider than the group shot on setting 10. Why that is, I have no idea.
As expected, neither pellet did especially well at 50 yards with the 1:22″ twist barrel. They did stay closer together than I expected, however.
The Premiers were more accurate than the JSBs, which parallels what both pellets did at 25 yards.
I don’t see any real evidence of tumbling pellets with either pellet on either power setting, so it’s too simple to say they’re just destabilizing. They’re less accurate but still stable at this distance. There’s probably something profound in that — something like the pellets still fly point-forward, but erratically. I can’t prove anything, yet, but now I have one barrel’s results in the can and it’s time to look at the factory barrel next. And that one has the twist rate that the manufacturer thinks is best for this airgun.
When I pitched the idea for this test as a feature article for Shotgun News, the editor told me he has never seen a test like this before. Neither have I. This may, in fact, be the first time anyone has published the results of testing three rifled barrels of different twist rates in the same gun under the same conditions. It probably has applications in the firearms world as well as for airguns. So, you readers may be in on something that’s being done for the first time.
We still have to test the factory barrel and the 1:12″ twist barrel at 50 yards. As a final report, I’ll summarize the entire test and the lessons I believe it teaches us.
by Tom Gaylord, a.k.a. B.B. Pelletier
Recently, we have had a number of questions about rifling twist rates that were attached to the twist-rate report. These questions are extremely important to the understanding of how bullets and pellets are stabilized, so I’m starting a tutorial on rifling twists today. I’ll keep adding sections as I see the need to explain more about the topic.
Today, I want to lay a basic foundation of what the rifling twist rate does. Blog reader Feinwerk asked if centerfire rifles (he said higher-power firearm rifles) had different twist rates than rimfire rifles, and the answer is yes. I’ll get to that, but let me start at a time when things were much simpler.
Early firearms shot multiple projectiles, similar to today’s shotguns that shoot birdshot and buckshot, but much cruder. It wasn’t long before people started experimenting with single projectiles. They found that single projectiles retained more of their initial energy than many smaller projectiles, so they did more damage when they connected with a target. The problem was getting them to connect.
After much experimentation, people discovered that spherical projectiles were the best for firearms. They flew the straightest because they didn’t have the irregular surfaces that created low-pressure zones to guide the bullet astray.
Then, rifling was discovered. Straight rifling (straight lands running parallel to the axis of the bore) was first used as a means of holding all the unburned gunpowder residue, of which there was much. That allowed the gun to be fired more times before cleaning. And, at some point, someone cut the grooves in a spiral — to make them longer to hold even more residue? We’ll never know for sure.
Once people saw how much straighter a spinning ball flew compared to one that was not spun intentionally, the race was on. For hundreds of years, the spinning round ball was the only bullet that was known. It reached its zenith as the patched ball used in the American rifle we know as the Kentucky — where the rifling doesn’t even engrave the lead ball, but spins it by spinning a cloth or leather patch that holds the ball tight while it’s inside the barrel. The ball gets very little distortion from the barrel — although there is a pattern around its circumference where the rifling pressed against the patched ball.
When the patched ball exits the muzzle, the patch falls away and only the bullet travels on to the target. Accuracy increased with this system, and the loading time dropped because the shooter didn’t have to engrave the lead bullet with the rifling when he loaded the bullet/ball.
The conical bullet
Things never stand still, though, and after the patched ball came into general use shooters began experimenting with bullets that were not balls, but rather longer cylinders of lead. These were the first conical bullets.
Although the round ball is very close to the same diameter as the conical bullet, it takes a lot more spin to stabilize the longer, heavier conical bullet.
A ball doesn’t need to spin very fast to be stable because its surface is smooth and regular. A conical bullet, on the other hand, is irregular — being longer than it is wide. Instead of a ball, it’s more like a spinning top that can balance only on its point as long as it spins fast enough. The longer the bullet, the faster it has to spin to remain pointed forward in flight. This attitude is called stability. If the bullet isn’t spun fast enough to remain point-forward, it’ll wobble like a top slowing down; and the varying air pressure that’s created will quickly cause it to tumble in flight. When that happens, the bullet will stray off its straight path.
This is when barrel makers began to be interested in the twist rates of their rifling. Prior to this time, they simply rifled the barrel with whatever twist rate their machinery supported. It’s a fact that the Hawken brothers rifled all their plains rifles with a 1:48″ twist, regardless of what caliber they happened to be.
With conicals, though, the twist rate does matter. Too slow and the bullet tumbles. Too fast and — well, less is known about what happens when the twist rate is too fast; but in my experience, you’re never able to get the same accuracy that you can when the twist rate is just right. A rifle that puts 10 shots into a half-inch with the right twist rate and bullet may put 10 of a different bullet that’s both lighter and shorter (and therefore both moving and spinning faster) into 1.5 inches.
The length of the barrel does not change the twist rate, nor its effect on the bullet — at least not directly. But a longer barrel sometimes does increase velocity. This is always true when black powder is used and can also be true when slower-burning smokeless powders are used.
A bullet that exits the bore of a barrel (of any length) with a 1:12″ twist rate and is traveling 1,200 f.p.s. is spinning at the rate of 1,200 revolutions per second (RPS). Speed that bullet up to 2,400 f.p.s. as it leaves the muzzle, and you increase the bullet’s spin to 2,400 RPS.
If a longer barrel causes an increase in muzzle velocity, it also causes an increase in the rotation rate of the bullet once it leaves the barrel. It does not change the barrel’s twist rate; but because the bullet is going faster; it’s also spinning faster. Reloaders take that into account when they load their cartridges. It’s possible to drive certain bullets too fast or too slow, resulting in less accuracy. Reloading is about finding a balance between the bullet and the velocity at which you launch it.
The most public and classic case of twist rates and their effects was the launch of the M16 rifle to the U.S. military. It addresses the specific question that Feinwerk asked. The early developers of the 5.56mm cartridge selected a twist rate of 1:14″ because the bullet was barely stable and would tumble and destroy flesh fast when it impacted a body. But they were focused only on the cartridge’s use in Vietnam — a conflict that was mostly conducted at short range and in very warm weather. The 1:14″ twist rate was too slow to stabilize the bullet properly beyond about 250 yards or in very cold weather. It worked great for a 40-grain .224-caliber bullet moving 4,000 f.p.s. from a .220 Swift, but was horrible when used with a 52-grain .224-caliber bullet moving 3,200 f.p.s from an M16.
Don’t confuse the caliber size in inches (.224″) with the name of the cartridge. Both the .220 Swift cartridge and the 5.56mm cartridge (M16) use the same .224″ bullets.
The twist rate for the M16 was increased to 1:12 inches, which worked better, but in time even that rate was discovered to be too slow to do everything the military wanted. Today, the twist rate for an M16 variant rifle runs anywhere from 1:7″ to 1:10″…depending on the specific model of gun, when it was made, which service owns it and what kind of ammunition it’s expected to shoot.
And the answer, Feinwerk, is yes…the twist rate of centerfire rifles does vary by caliber, by the bullets used and the velocities at which they’re driven.
This first twist-rate primer report was written at a very high level. I don’t know whether or not it addresses everything you wanted to know, so I’ll read your comments with interest. If we need to go into greater detail, that’s always possible. Otherwise, I’ll remain at this overview level in the next report.
by Tom Gaylord, a.k.a. B.B. Pelletier
Before we begin, there was a request last week for me to test the Benjamin 392. I thought I’d tested it already, and it turns out I did. Click see all 5 parts on the old blog.
The question of barrel equivalency
Today, we’ll look at the accuracy side of this test, where yesterday we looked at how the twist rate affects velocity. Before I begin, however, we have to settle an issue that’s in a lot of people’s minds. Namely, is it reasonable to test barrels made by Dennis Quackenbush against a Lothar Walther factory AirForce barrel? Will the test results be skewed for that reason and not because of the different rifling twist rates? Or will the twist rates determine part of the results and the barrel’s pedigree determine the rest?
To carry that kind of logic out to its conclusion, every barrel will be different. Who’s to say that one Lothar Walther barrel is not better or worse than another? The fact is that we know that some of them will stand apart — being either better or worse than the norm. There’s no way to test the exact same barrel with all 3 twist rates because we cannot adjust the twist rate of a rifled barrel. Once it’s built, it is whatever it is. Three separate barrels have to be used for this test, whether they’re made by Lothar Walther or by someone else.
Another thing to consider is that other smallbore barrels made by Dennis Quackenbush in the past have proven to be as accurate as barrels made by Lothar Walther. I’m now talking about barrels of the same caliber — not comparing .22-caliber Walther barrels against .308-caliber Quackenbush barrels. I know of .22-caliber barrels and .25-caliber barrels made by Quackenbush that can go against the best barrels made by Lothar Walther.
But in the end, we can never know if the barrels I’m using for this test are exactly as accurate as the Lothar Walther factory barrel I am also using. There is just no way to know that. There is, however, a way of getting pretty close. If the Quackenbush barrels produce some groups in the same accuracy range as the Lothar Walther barrel, then we can be assured that we are testing barrels of similar quality. We can never be entirely certain they are exactly as good, so this is as close as it is possible to come. And, by the same logic, that is all we could do if Lothar Walther had made all 3 barrels used in this test.
Lothar Walther is not going to make test barrels for a test like this unless they want to. There is no way to contract with them to make one-off barrels that have the different twist rates we are testing. They don’t make one-off barrels — at least not for the public. There are barrel makers who do make one-off barrels, and Dennis Quackenbush is one of them. We’re going to have to be satisfied with the barrels he has provided and look at the results to see if we believe his barrels are as good as the factory barrel we’re testing.
On with the test
With that issue set aside, let’s look at what the 3 barrels did in this test.
The group sizes in the table are shown in inches.
The first thing I looked at was the results of the 2 Quackenbush barrels against the factory Lothar Walther barrel, and I learned something interesting. At 10 meters, the 1:22″ twist barrel shot better than the factory barrel with Crosman Premier pellets — not by a little, but by a significant margin. On both zero and 6 power, the 1:22″ barrel made groups that were one-tenth of an inch smaller than those made by the factory barrel. On power setting 10, the factory barrel beat the 1:22″ barrel by only four one-hundredths of an inch. I think that’s significant. It shows that the 1:22″ twist barrel can keep up with and even exceed the Lothar Walther barrel…at least to 10 meters.
The 1:12″ twist barrel wasn’t quite as good as the factory barrel at 10 meters, but it was very close. At power settings zero and 6 they differed by just a few thousandths of an inch. On setting 10, the factory barrel exceeded the 1:12″ barrel by the same four one-hundredths of an inch that it did with the 1:22″ barrel.
These results tell me that the two barrels made by Dennis Quackenbush are as capable of making good groups as the Lothar Walther barrel. The differences in the group sizes are less than the measurement errors made when measuring the groups. That puts the barrel equivalency question to rest in my mind. I’m willing to accept that the results of this test are from the different twist rates and not from the inherent quality or lack of quality of the barrels being tested.
Ten meters is really too close for definitive results when the question of accuracy is on the line. That’s why the test was also shot at 25 yards, and later I’ll shoot it at 50 yards. It was at that distance that the factory barrel really shined. With 14.3-grain Crosman Premiers, the factory barrel was more accurate on power setting 10 than on the lower settings. With 15.9-grain JSB Exacts, the factory barrel did its best on power setting 6. But both of the other barrels did not perform their best at this range.
The 1:12″ barrel shot Crosman Premiers best on power setting 6, but it shot JSB Exacts about the same on settings 6 and 10. There was a little difference with the Exacts, but it was down in the measurement error margin.
The 1:22″ barrel, on the other hand, got worse with Premiers as the power was increased. And the difference between the best and worst groups was large enough to be more than a coincidence. It was almost twice as large on power setting 10 as it was on power setting zero (1.082″ compared to 0.671″).
With JSB Exact pellets, that trend was reversed. The greater the power, the smaller the groups became. For both pellets at 25 yards, the 1:22″ barrel was not anywhere near the factory barrel at the same distance. Only at power setting zero with Premiers was that barrel close to the factory barrel, and the difference was still outside the measuring-error margin.
The 1:12″ barrel performed similar to the 1:22″ barrel at 25 yards. Only once, at power setting 6 with Premier pellets, did the 1:12″ barrel exceed the factory 1:16″ barrel.
If you look at the results of the 1:22″ barrel, you’ll note that it falls apart at 25 yards. It held its own at 10 meters; but at 25 yards, it falls far behind the factory barrel.
The 1:12″ barrel does the same, but it doesn’t open up as much as the 1:22″ barrel. There are individual instances where one of the two Quackenbush barrels exceeds the other; but in general, what I’m saying holds true. The factory barrel, however, doesn’t work that way.
The factory barrel holds together at 25 yards, just as it does at 10 meters. Again, there’s a single instance at 25 yards where it is beaten by the 1:12″ barrel (power setting 6 with Premier pellets), but that stands out as the only time the factory barrel was beaten at 25 yards. We can say in this test the factory 1:16″ twist barrel outperformed the other two barrels at 25 yards. That allows me to say two things.
First, I predict that the factory barrel will be the most accurate at 50 yards. Second, I believe that the 1:16″ twist rate is the best twist rate of the 3 we’ve tested.
Don’t get too excited
This test has been too small to say anything with certainty. I said that before we began, and I’m saying it again now. All these results can do is suggest things that should be looked at more closely. Other pellets will give different results — I’m sure of that. And more testing will refine these numbers. Where there’s gross separation between the Quackenbush barrels and the factory barrel, I feel certain the trend will continue with more testing. But where things are close, there might be a reversal of the outcome. There simply isn’t enough test data to say otherwise.
Conclusions so far
Based on the data we see thus far, I think that the 1:16″ twist rate seems to be the best of the three twist rates we’ve tested. At 10 meters, the factory barrel shot 3 of the best 6 results. At 25 yards the factory barrel shot 5 of 6 groups better than the other 2 barrels. Out of the total of 12 results, the factory barrel was the best 8 times. So, 67 percent of the best groups in this test were shot by the factory barrel.
The factory barrel is the best overall barrel in this test so far, and it gets better the farther the distance is to the target.
Because the other 2 barrels beat the factory barrel one-third of the time, I think the question of their quality can be laid to rest. Clearly, they’re giving the Lothar Walther factory barrel a run for the money.
Power versus accuracy
In Part 8, I talked about not needing to use power setting 10 with the 1:22″ barrel if it proved to be accurate enough. The velocity at power setting 6 was so high that I said I could stop there and conserve air. I think I’ve now shown that it isn’t accurate enough. Yes…when you look only at that barrel’s accuracy, you could surmise that it’s accurate enough at power setting 6 to stop at that point. But when the factory barrel is brought into the equation, the 1:22″ barrel falls behind.
Next, I’ll shoot all 3 barrels at 50 yards. I’m planning to shoot only power settings 6 and 10 with each barrel because of the distance involved. This will be outdoors, and the pellets will really scatter in the light breezes if they’re shot on the lowest setting. I will have to wait for ideal range conditions, so it may be quite a while before I’m able to make my next report.