Posts Tagged ‘scope level’
by Tom Gaylord, a.k.a. B.B. Pelletier
This report covers:
• Why does a scope need to be leveled?
• Scopes cannot be leveled.
• A leveling solution.
• Bubble levels.
• What works
• What about a collimator?
• My moment of enlightenment.
I promised this report to blog reader Genghis Jan over half a year ago. Several times, I’ve started to write it and turned away, but today I’m seeing it through.
Why level a scope?
There are 2 reasons for leveling your scope. The first is psychological. If the reticle inside the scope appears to be slanted to one side when you mount it on your gun — and I am primarily talking about rifles today, although these principals apply to scoped pistols just as well — it’s disconcerting. The second reason for leveling a scope is to ensure the vertical adjustments move the strike of the rounds vertically, and the horizontal adjustment do the same. If the scope does not appear level, the adjustments will move the rounds off to one side or the other as they move up and down.
If a scope’s reticle appears tilted, do you then tilt the rifle sideways to level the reticle? Or do you just hold the rifle so it always “feels” level and tolerate the reticle that seems tilted? I’ve tried both, and neither one is as comfortable as having the scope aligned perfectly — so it appears level when the rifle is held comfortably. But, just so you know — both solutions will work because there’s no such thing as a level scope.
Rifle scopes cannot be level
Sorry to disappoint, but there’s no scientific way to ever level a scope to a rifle — at least not to any rifle that has ever been made. I used to be a tank commander; and on occasion, there are reasons to level the tank cannon. But there’s nothing on a tank cannon that is level. So the makers did something about it. They machined several pads on the breech of the cannon where a precision level can be stood. This level — called a gunner’s quadrant — has “feet” that are steel pads machined into it. Their purpose is to stand the quadrant on the machined pads of the cannon’s breech and establish a level.
Without these machined pads, there can be no point on the cannon that is level. What I’m saying is that there’s no spot on the mechanism that’s true enough that a bubble level placed upon it would have any meaning. It might be possible to get the bubble centered on the gunner’s quadrant, but you could never be sure of doing it again with the same results. But with the machined pads, you have what you need — a reference point — to declare the gun to be level.
Level is a relative term
You see, the term “level” relates to the earth. An item can lay on apparently flat ground and not be level according to its bubble or the center of its plumb bob. Reference points are needed. And no rifle I have ever seen has them. Therefore, no rifle can ever be level! Please think about this before you comment.
I’ve used several leveling solutions in the years I’ve been mounting scopes. One was to hang a plumb bob from a target backer at 50 yards and align the scope’s vertical reticle with it once the scope was mounted on a rifle. I thought this would guarantee that the scope was level. Maybe it did, but it often put the scope at odds with the rifle because the rifle’s scope base was not machined so the scope appeared level when aligned this way. I actually shot several rifles with what appeared to be a slight cant in the scope reticle because I had aligned the scope in this fashion. The things that crazy people will put up with to maintain the universe they create!
A plumb line has been suspended from a target backer at 50 yards (left). Now, the scope (right) will be superimposed upon it and the vertical reticle will be aligned with the plumb line.
The scope has been shifted over the plumb line and rotated in the rings until the vertical scope reticle is aligned with the plumb line.
Neat trick! Is the scope aligned? Yes, it is. Is this the best way to do it? No, it isn’t. Let me show you why.
Now, this is what you may see when you hold the rifle to your shoulder. It may not be as obvious as this, but unless the scope mount was machined exactly with regards to the plumb line, you’ll notice the tilt and it will bother you. Are you then supposed to hold the rifle “level,” or do you “level” the reticle before shooting?
I’m showing you what happens when you attempt to level the scope by outside means. Sometimes, those processes guarantee the scope will not appear level when you hold the rifle in your arms.
Another method that doesn’t work
Some folks will purchase a bubble level for their guns, mount it and declare the job done. Of course, there’s no reference point on the gun that they can refer to, which is why I told you about leveling tank cannons at the start. Doing it this way is like driving a car in dense fog by keeping the fenders between the turn signals!
The bubble level is a wonderful tool but not for leveling scopes! It’s for leveling shooters in the field. Once the bubble is level, you know the scope is aligned the same as when it was sighted in on the bench (as long as the bubble was level at that time).
Here are two different kinds of bubble levels being tested on my Whiscombe rifle. They help to level the rifle in the field — not to level the scope during mounting.
Genghis Jan — this is the only way to level a scope. Mount the scope on the rifle and turn the tube until the vertical reticle appears to bisect the rear of the rifle.
You extend the vertical reticle line downward and see how it looks against the back of the rifle. When the vertical scope reticle appears to bisect the centerline of the rifle, the scope is level.
You can argue that this is an imprecise method of doing this, and I cannot defend it. Because — as I have already shown — there is no such thing as a level scope. Level with what?
But here is what I do know. The scope will never look right until you level it this way. If you pick up the rifle and the scope appears to be canted to one side, how does that make you feel? It’s that feeling that either inspires confidence or instills doubt, and these two emotions will drive you to success or failure while shooting.
As you gain experience with your rifle, your hold may change, and you may need to adjust the scope, again. After that, you probably have to sight-in once more. I’ve had this happen to me several times with different rifles.
What about a collimator?
You’ve heard of an optical device called a collimator that magically aligns scopes for rifles? Gun stores use them, don’t they? Yes, they do, but not to align the scope reticle — not to level the scope. They use a collimator to align the axis of the barrel with the scope’s line of sight — so your bullets land on paper at 50 or 100 yards when you go to the range.
But an airgun can be sighted-in at ranges much closer than 50 or 100 yards. I usually start at 10 feet. I wrote an article about sighting-in this way that you really should read. Adjusting a scope so it looks level has nothing to do with sighting-in unless the scope does not appear to be level to you, and then you won’t be able to adjust it properly.
My moment of enlightenment
You may well ask why it took me so long to write this report, when it seems so simple (and really is!). It is because I know that the simplicity of this will offend many people who believe that leveling a scope must be a complicated procedure. Some will refuse to believe me and will insist they level their scopes by the methods I’ve written about or by some other process.
I believe that this step is why many buyers shy away from buying scopes for their guns unless they are mounted by the factory. Well, guys, I used to BE the factory! That’s right. For three years, I mounted every scope that was mounted on an AirForce air rifle and then I zeroed it. I learned how to mount scopes the best way, which is also the quickest way, and additionally it’s the only way that will ever satisfy the user of the rifle.
I’ve had to adjust scopes (rotate their tubes in the rings slightly) that others have mounted to get their crosshairs in line with the vertical axis of the gun. If you’ve owned many guns with scopes, I’m betting you’ve done this, too. When you do, you’re simply doing what I said to do several paragraphs previously.
I know this sounds too simple and too unsophisticated to work; but believe me, this is the only way it does work. As you gain experience with scoped airguns, the truth of this report will become increasingly evident.
by B.B. Pelletier
The FWB 300S is considered the gold standard of vintage target air rifles.
This is a test I said I would do the next time I got a calm wind day at the range. That day came last Friday, and I took the opportunity to test the FWB 300S at 50 yards with a scope. This test was designed to see if there is any discernible accuracy difference between pellets that are sorted by weight and those selected at random from the tin. If you read part 4, you’ll see that I was surprised to find that these JSB Exact RS pellets I selected for their accuracy had such a variation in weight. I sorted through almost 40 pellets to find 20 that weighed exactly 7.3 grains. Though the weight difference was only four tenths of a grain, it was more than expected and more pellets were affected than I thought.
The JSB Exact RS pellet was chosen because of previous performance demonstrated in part 3. And I had to choose a domed pellet because out at 50 yards no wadcutter can possibly be accurate — I’ve proven that on many occasions in the past.
In part 4, I tested the rifle at 50 yards using the target sights that come on it, and I got two groups of 10 shots each. One was with random pellets taken from the tin. That group measured 1.689 inches between the centers of the two widest shots, while the other was 10 weight-sorted pellets that grouped in 1.363 inches. I didn’t feel that test was conclusive, so I wanted to return with the rifle scoped to see what it could do.
Not only did I mount a scope on the rifle, I also installed a scope level, and on every shot the bubble was leveled. That eliminated the possibility of any cant, so the rifle was always shooting in the same orientation.
The scope hangs over three-quarters of the loading port, making loading a chore. Notice how close together the scope rings are, yet they occupy the entire length of the dovetails. The 300S is not made for a scope! Notice, also, the scope level that was consulted on every shot.
I mounted a Leapers 3-9×50 scope with AO. It’s an older version of the one I linked to, but the specs are mostly the same. Notice in the photo that this scope was almost too long for the rifle, even though it was mounted at the extreme rear of the spring tube.
Where I had used a 3-inch bull target with the aperture target sights, I switched to the smaller 10-meter target when using the scope. The pellets were falling off the target paper anyway and onto the plain backer paper attached to the target frame, because of the large drop of this pellet at 150 feet.
I couldn’t have asked for a better day in which to shoot. Since I was at the range very early, there was absolutely no breeze. The sun hadn’t risen very high, so I didn’t need to shield my non-sighting eye. The rifle rested in the bunny bag dead calm, so altogether this was as perfect a test as I could have run.
Bore already seasoned
Because the bore had been shooting JSB Exact RS pellets last, it was already seasoned for this test. Still, I did shoot the rifle a few times to wake up the action. Then, I began the first group of unsorted pellets.
This time, the pellets did very poorly — grouping 10 shots into 3.152 inches at 50 yards. The group is very elongated, looking like a large velocity swing. The group measures just 1.178 inches wide, which is less than half the height.
Next, I shot the pellets that were sorted by weight. Ten went into a group measuring 1.606 inches across. This group is fairly round and well-distributed, so it makes me wonder all the more about the first group. Perhaps the gun needed longer to warm up for the first group than I allowed?
Test is not conclusive
I’m declaring this entire test invalid. I think I’ve stretched the FWB 300S beyond its capability, and the results are not telling me what I need to know. I’m aware that others have shot 10-meter rifle at 50 yards and say they’ve gotten good results, but clearly I’ve not been able to do the same with this rifle.
I think the test itself is worth pursuing, but with a rifle better-suited to accuracy at 50 yards. Pushing the FWB 300S outside its comfort zone was not a good idea. But I have several accurate air rifles that are all capable of grouping well at 50 yards. That’s what I need to rerun the test.
by B.B. Pelletier
Before we begin today’s report, I want to reflect on a truth that helps explain why we’re sometimes dissatisfied with things when we get them. I was at the range last week testing some airguns, and I was using my Celestron 70C spotting scope, like I always do. This scope is good at 50 yards, but becomes marginal at 100 because it cannot resolve the smaller bullet holes. I don’t mean pellet holes, I mean .22-caliber bullet holes in the black bullseye at 100 yards.
A friend brought his Burris spotting scope to the range for me to compare, and the difference between the two was night and day. His scope was sharp enough to see small spiders walking on the target paper at 100 yards!
That same day, I shot a couple rifles my other friends had brought to the range. One of them was a fantastically accurate custom 6mm/.22 that can shoot a half-inch, 5-shot group at 100 yards. But it’s scoped with a BSA 4-24X scope that isn’t available at Pyramyd Air for good reason. It’s so dark and muddy that I couldn’t see the bullet holes as I shot this very accurate rifle. And I was shooting at a bright orange dot on white paper! I couldn’t see bullet holes on that! The scope was set at 16X because anything above that caused the image to go white from flare, and we were shooting in total shadow under a covered roof!
Another friend had a Lyman Super Targetspot scope that was 10X, and I could easily see all the holes his .220 Swift was making at 100 yards on the same kind of targets. So with six fewer magnifications, I was able to see the holes because of the superior optics. The Lyman scope is no longer made and a good used one will fetch about $800, while you can probably buy the BSA for under $200 all day long.
Back to the spotting scopes
My Celestron spotting scope has 30X, 60X and 90X power available. Where do I have it set? Between 30X and 60X, which is about 45X. Because anything greater than that muddies up the image too much — even at 50 yards. My friend’s Burris spotting scope only goes up to 45X maximum, yet it’s many times clearer than my Celestron. And I found on this day that only 10X was actually required to see .22-caliber holes at 100 yards on a light target. A black bull is probably harder to see with just ten power, but with 45 power it is easy!
So, I went home and researched that Burris spotting scope. It sells for about $175 at Midway, where the Celestron C70 Matsukov I have sells for about $80. But what good is it if it doesn’t do its primary job?
That got me thinking about shooters who purchase air rifles on the basis of velocity, alone, without appreciating all the other variables that come in the package. Things like smoothness, a good trigger and, most importantly, accuracy are pushed aside for velocity and low price.
I rant on about these things because they are also among my personal flaws. I’ve always tried to buy the cheapest of everything, and often the “mostest powerfulest,” too. So, I’m constantly drawn back to them whenever I find that I have made another dumb blunder.
On to today’s report
This was a day of great learning. When I summarize this series of tests, today will mark a big turning point, I believe. I learned a lot of things — all of which I will now show you.
As always the first pellet tested was the lightweight Beeman Devastator. I’ve been amazed at the performance of this pellet from the start of this test, and last time I predicted that it would shoot even smaller groups this time.
Sorry, but that didn’t happen. The first four shots seemed to confirm the need to “season” the bore before shooting each new pellet. I’m showing that target here so you can see what I saw.
This sight-in target for Beeman Devastators fooled me. Shot one was the upper right. Shots two and three were in the same hole on the left and shot four was between that group and the first shot. At the time, I felt this was demonstrating the need to “season” the bore before shooting a group with this pellet.
Shot one was the on at the high right, then the tight group of shots at the left were the next seven shots. Just when I was about to proclaim a universal law of bore seasoning had been discovered, shots nine and ten fell between the first shot and the group at the left.
Now, I was in a quandary. If the last two shots had strayed from the group in the same direction as the first shot, what was it telling me? Maybe the bore didn’t need seasoning. Maybe the Devastator just doesn’t like this velocity. Only another group would tell me for sure.
I believe that this particular velocity is one the Devastator pellet doesn’t like. As you know, I haven’t touched the Harmonic Optimized Tuning System (HOTS) on the barrel of my Whiscombe during this test. And I won’t. I don’t really care how accurate the gun is with a certain pellet. I was trying to see if there is a linear relationship between velocity and accuracy, and these two groups suggest that there isn’t. You’ll have to read the rest of this report to fully understand what I mean.
Crosman Premier lites
Next, I shot 10 Crosman Premier lites at 25 yards. This time, the target was very good, but the point of impact moved about a half-inch higher than last time. I was prepared to shoot four shots to season the bore, but the pellets kept going through the same hole, more or less, so I just completed the group without any seasoning.
This is a nice group of 10 Crosman Premier 7.9-grain pellets at 25 yards. It measures 0.747 inches and is actually slightly larger than the smallest group of Devastators. It’s almost identical to the last group shot during the previous test, so no big change in group size with this pellet.
The change of impact point was another big lesson this time. I’d seen it earlier, but not as dramatically as this time. The overall size of the group remained the same as before. That’s another indication that this is a barrel vibration issue and not one that’s driven by a change in velocity.
The next pellets I tested were the Beeman Kodiaks, which have performed so well to this point. This time, the point of impact shifted up about three-quarters of an inch, but the group remained very tight. This time, 10 shots went into a group measuring 0.472 inches at 25 yards, which I believe is currently the best group of this entire test. Last time, they gave us a group measuring 0.628 inches.
And, now, it’s time to report the performance of the super-heavyweight 16.1-grain Eun Jin pellet. Last time, they made the worst showing and that continued into this test, as well. What was different was the fact that the point of impact dropped more than two inches with what was just a slight reduction in velocity.
I was aiming at the center of the bull above, where they impacted! This was a dramatic change of impact point from the last test. Group size for these 10 Eun Jin pellets was 1.27 inches between centers. That’s a little larger than last time, but the change of impact was far more noticeable.
What have we learned?
Well, nothing has been linear in this series of tests — the way I expected. But what the vibrations are doing seems pretty obvious. So, the next test is both needed, as well as anticipated.
by B.B. Pelletier
For the benefit of readers who have landed on this article first, this is the sixth test in a series of reports designed to test how velocity affects accuracy. I’m using a Whiscombe JW75 breakbarrel/underlever rifle with a .177-caliber barrel installed. That way the same powerplant is being used for each test. I’m controlling the power of the gun by the use of different air transfer port limiter screws that allow less and less air to past through.
The Whiscombe rifle uses dual opposed pistons that come together to compress the air when the gun is fired. The rifle has no recoil and just a minor impulse that can be felt — yet it’s one of the most powerful spring-piston air rifles ever made. My rifle can produce over 30 foot-pounds in .25 caliber.
Four pellets were chosen at the start of this test and have been used in each velocity and accuracy test that’s been conducted. Two of them were supersonic in the first test and one remained supersonic in test two, while the other was still in the transonic region where wind buffeting occurs. It’s testing exactly what it was designed to.
Today, I installed a smaller transfer port limiter to slow down the four test pellets even more. This was the first time all four pellets were safely below the sound barrier, and the shooting was uniformly quiet. You probably should read the first five reports at the links provided above to understand all that’s happening.
This 7.1-grain lead hollowpoint hunting pellet has been the biggest surprise of this entire series. Normally, these types of special pellets are designed for maximum mushrooming and are not that accurate — at least not in my experience. But the Beeman Devastator has proven to be the exception. From the start, when it was leaving the muzzle at an average 1,216 f.p.s., it was accurate. So far, we’ve conducted only two accuracy tests, so the results of this one should prove interesting. As I said, this was the first time this pellet has left the muzzle at subsonic speed.
The average velocity was 973 f.p.s., with a low of 967 and a high of 977. That’s only a 10 foot-second difference between the slowest and fastest pellet, which I think is amazing. Usually, a pellet this light will also have a much larger total velocity spread over a 10-shot string. At the average velocity, this pellet is cranking out 14.93 foot-pounds of energy at the muzzle. That’s down from the initial 23.32 foot-pounds it was making when there was no transfer port limiter in the gun.
Can you guess what this pellet will do in the accuracy test? I think it’ll be even more accurate than in the past. But who knows?
Crosman Premier lites
Crosman Premier lites were next. Initially, they were leaving the muzzle at 1,134 f.p.s. when there was no transfer port limiter in the gun. Today, they averaged 915 f.p.s. and the spread went from 911 to 919, for a super-tight 8 foot-second difference. At the average velocity, these pellets were producing 14.69 foot-pounds at the muzzle.
Any guesses as to what this pellet will do in the accuracy test? The difference between the first and second accuracy tests was very small, but we’ve finally gotten the velocity down out of the transonic region, where all the buffeting is. They didn’t break the sound barrier the last time; but at an average 1,057 f.p.s., they were well into the transonic range. There could be a happy surprise from the Premiers.
Beeman Kodiaks averaged 819 f.p.s. with this transfer port limiter. The spread went from 817 to 824 for just a seven foot-second total variation across 10 shots. The muzzle energy was 15.2 foot-pounds. Notice that this heavy pellet is producing more energy than the two pellets that are lighter — something that’s not supposed to happen with a spring-opiston system.
I need to interject a word on Kodiaks. For some reason, their weights have varied widely over the past couple years. Where they had weighed 10.6 grains for several decades, someone decided to reduce the weight to 10.2 grains in 2010. Those are the pellets I have. But now I notice that the weight is back up to 10.4 grains. Obviously, someone is adjusting the weight of this pellet that was once considered one of the three most accurate .177 pellets on the market.
Kodiaks have been the most accurate pellets in both the previous accuracy tests (after I started using the scope level), but I don’t know if they’ll continue that trend at this new lower velocity.
Eun Jin domes
The 16.1-grain Eun Jin dome is really too heavy for the Whiscombe in its most powerful form. Certainly as the power is reduced, they become even less appropriate. In this test they averaged 618 f.p.s., and the spread went from 615 to 624, for an 11 foot-second total. At the average velocity, these super-heavyweights are producing 13.7 foot-pounds at the muzzle.
They produced the worst group last time, opening up from the group they had made with no transfer port limiter installed; and I predict this trend will continue in the next accuracy test. These are really best in powerful PCP guns — not in spring-piston guns.
Next, I’ll test the accuracy of these four pellets at 25 yards. I’ll use the same careful technique that I’ve been using so far to keep the results as free from bias as possible, so this should give us a good look at how velocity really affects accuracy — at least with these four pellets in this one gun.
You may not remember this, but at the beginning of this series I said that if the results of three velocity and accuracy tests seemed to warrant it, I could do a fourth test with the velocities cranked down very low. I’ll hold off on the decision to do that until I see the results of the next accuracy test.
Whether I do a fourth velocity/accuracy test or not, I’ll write a final summary report on all that’s been done in this test. If it seems worthwhile, I could do a second test using a Talon SS, so we can see the same relationship in a PCP gun. However, that hasn’t been decided, yet. We still have to get through this one.
by B.B. Pelletier
Today we look at the groups made by the four pellets used in this test at 25 yards when the velocity is diminished. Part 4 covered the velocity for each pellet, so go there to see where each one is.
If you like nice linear results, prepare to be disappointed. Today’s target do show changes, but they may not be in the direction you expect. Let’s get right to it.
First up were the 7.1-grain Beeman Devastator pellets. These pellets have been a real surprise in this test, because they have proven to be accurate at supersonic velocity and they are not sensitive to bore conditioning. Shoot one and it tends to go to the same place every time. They also do not show any first-shot tendencies that so many other pellets do. This is a real plus for hunters, who are always shooting the first shot. I will have to return to this pellet sometime soon and test it in other guns, because it seems to be a real winner.
This group measured 0.615-inches between centers, compared to the first group in Part 3 that measured 0.743-inches. Obviously the slower pellet is significantly better, but because we only have two groups to compare it is impossible to know whether one will always be better than the other. The Devastator is still going out the muzzle at supersonic speed, even though it has been slowed by about 70 f.p.s. This is the second-best pellet of this test, just as it was in the first test.
All the shooting conditions are exactly as reported in Part 3. The scope level was used for every shot and the rifle was held in the artillery hold, with the stock resting on my hand at the same place every time. So the same care is being taken each time I shoot, and that won’t change.
Next I tried the 7.9-grain Crosman Premier lites. The bore was conditioned with four shots before shooting the group shown here, and as before, the Premiers did show some wandering in the first three shots. After that, they seemed to settle down, though as you will see, settling is a relative term.
This time the group size remained nearly the same, at 0.754-inches. The group shot when the gun was shooting them faster measured 0.778-inches, which is too close to call. Yes, the group fired with the pellet going slower is a trifle smaller, but it is really too close to say there is a difference that’s due to the velocity — especially given the built-in error in measuring groups that I have talked about.
The third pellet I tried was the heavier Beeman Kodiak. They turned in the best group of the test, just as they did the first time. And the ten-shot group measured 0.628-inches between centers, which is very close to the 0.633-inches of the first test.
I cannot say that the Kodiaks did any better this time than they did in the first test. Even though the velocity was slowed by 55 f.p.s., it doesn’t seem to make much difference at 25 yards. Maybe that’s just not enough of a drop to matter.
The last pellet tested was the 16.1-grain Eun Jin dome. These pellets were never in the transonic region, even with no transfer port limiter in the gun. So this time they were just going slower. And what a difference that made!
Ten Eun Jin pellets were all over the place at 25 yards. They made this huge group that measures 1.118-inches between the centers of the two pellets farthest apart. This is the worst group of this test and also much worse than the first test with this same pellet, when they grouped 0.798-inches.
Eun Jins made a bad showing at 25 yards with the transfer port limiter installed. As long and heavy as they are, I guess they need all the velocity they can get, to obtain the rotational speed required for stabilization.
What have we learned?
The first thing we learned is this probably was not enough of a velocity drop to matter, except for the slowest pellet. The next thing we learned is that the changes are not always linear. Although three of the pellets exhibit what I would call a linear relationship of accuracy to velocity, only one of them — the Beeman Devastator — shows any real improvement. And even that is just a very small improvement that could just be due to chance.
The next thing we learned is that some pellets need their speed to maintain accuracy. The big heavy Eun Jins do not like this lower velocity, and I will predict they are going to get even worse with the next velocity decrease.
The Crosman Premiers are still not performing well, and since they are still within the transonic region, there is good reason for that. I look for an improvement in the next test.
What all this tells us is that using very long heavy pellets in a lower-powered airgun is probably not a good idea. When the distance to the target increases beyond about 30 feet, you will start seeing these pellets spread out, where lighter pellets will probably continue to be accurate.
by B.B. Pelletier
Today we lower the velocity of the Whiscombe and test each of the four pellets, in preparation for the next accuracy test.
One reason I selected the Whiscombe for this series of tests is the fact that I can control the power output over a wide range of velocities by installing various transfer port limiters. For those who are new to airgunning, every spring-piston airgun like the Whiscombe generates a brief blast of compressed air by means of a piston racing forward in a compression tube. In the Whiscombe’s case it is actually two pistons racing towards each other. At the exact end of their travel a small air tunnel called an air transfer port conducts the compressed air from the compression chamber to the base of the pellet, where it blows it out the bore.
The amount of compressed air is extremely small, but the pressure is fantastically high for a brief moment — as much as over 1,000 psi. In fact, well over 1,000 psi. Now you probably also know that precharged airguns operate on compressed air that is at an even higher pressure, but the difference is, when a precharged valve opens, it does so relatively slow, releasing air at far below the pressure that’s in the reservoir. But a spring piston gun doesn’t have any valves, so when the air is compressed, it flows through the air transfer port to the back of the pellet like a small explosion. Everything the gun has to give is right there at the same time.
However, if anything gets in the way of this air as it flows, it slows down the flow and the resultant energy output of the gun. The amount of compressed air remains the same, but the rate at which it flows is slower, and the pellet is not moved with the same sudden push.
That’s a fancy explanation for the Whiscombe’s air transfer port limiters, which are nothing more than Allen screws that screw into the threaded transfer port. In the center of each are holes of various sizes though which the air must then pass.
The rifle has a threaded air transfer port. It’s shown here without any limiters installed.
These limiters have various sized holes that reduce the airflow through the transfer port when they are installed. For this test I will install the largest one on the left.
The transfer port limiter is installed with an Allen wrench.
Now let’s see what changes have been made by the insertion of the transfer port limiter. I will shoot the same four pellets as before and in the same order.
First I tested the Beeman Devastator pellet. This is the pellet that gave an average velocity of 1,205 f.p.s. in Part 1 without any transfer port limiter. With the limiter I show being installed above the Devastator’s average velocity dropped to 1,123 f.p.s. In the first test the spread was 28 f.p.s. In this test is was 16 f.p.s., running from 1,116 to 1,132 f.p.s. That isn’t a huge drop in speed, but the power that was 23.32 foot-pounds in the first test has dropped to 19.89 foot-pounds. We will see what this does to the accuracy.
Next came the Crosman Premier lite pellet. In the first test they were averaging 1.134 f.p.s. for an energy of 22.56 foot-pounds. The spread in that test was only 12 f.p.s. In this test the same pellet averaged 1,057 f.p.s. for an energy of 19.56 foot-pounds. The spread was an even tighter 6 f.p.s., running from 1,054 to 1,060 f.p.s. That is incredible performance at this power level.
Beeman Kodiaks came next and they averaged 992 f.p.s. in the first test. That generated an average 22.29 foot pounds of energy at the muzzle. The spread in test one was 22 foot-seconds. After the transfer port limiter was installed the average velocity dropped to 937 f.p.s. for an energy of 19.89 foot-pounds. The total velocity spread was 24 f.p.s. — ranging from 927 to 951 f.p.s.
The heavy 16.1-grain Eun Jin was the final pellet I tested. In the first test they traveled 726 f.p.s. and generated 18.85 foot-pounds of energy. The spread was 13 f.p.s. With the transfer port limiter installed they averaged 687 f.p.s. for an energy of 16.88 foot-pounds. The spread with the limiter installed was 10 f.p.s. — from 682 to 692 f.p.s.
Next I shoot groups at 25 yards with each of these pellets so we can compare them to the groups fired with no transfer port limiter. That will give us our first look at how velocity affects accuracy.
After that I install another limiter that restricts the airflow even more, so the rifle shoots slower. And then we shoot it at that level for accuracy.
At the end of this test I still have one pellet out of four that’s supersonic. The Devastator, at 1.123 f.p.s. will always be supersonic where I live. The Crosman Premier, however, will be right on the cusp of the sound barrier and whether of not it breaks though will depend on the day.
Normally I would run the accuracy part of this report immediately after the velocity report, but something wonderful happened at the range on Wednesday. So I am making a special report on Friday to show you something very amazing — at least in my eyes. And no, it isn’t the Ballard. Not yet.
by B.B. Pelletier
I’m writing this report because I saw from the comments on the accuracy versus velocity test that several readers do not know what a scope level does. And where three people speak out, there are three hundred who are reading and remaining silent.
They say that there’s nothing more zealous than a convert, and I expect that is true of me when it comes to scope levels. I have understood their need for a long time and even conducted a fairly extensive cant test back in my Airgun Letter days, but it was my .38-55 Ballard single-shot rifle that really drove the message home. That rifle came with a bubble level, and it’s far more precise than the levels we find on air rifles today. The bubble moves very slowly, making it important to check the level just before you begin the squeeze; because what looks like a level gun one moment can change slowly to a canted gun if you don’t watch the level. By contrast, the scope levels I’m using with airguns have bubbles that move very fast, are much easier to see and are far simpler to work with.
Today, I want to demonstrate the effects of using a bubble level. I’ll use the same Whiscombe JW75 that I’ve been using for the velocity versus accuracy test, because we already know it has an accurate pellet in the Beeman Kodiak. For this test, I first seasoned the bore with six shots, then fired a group of pellets with the bubble deliberately off-center in both directions. I fired a second group where I paid no attention to the level and just tried to level the rifle as best I could through the scope. The final group was shot using the level with the gun absolutely level for each shot. The distance was 25 yards, which several readers mentioned is almost too close to see the effects of using a level.
This is the insidious part of leveling a gun, and it’s what I’m showing with today’s test. You really can’t see a pattern to the group from not leveling the gun when you’re shooting as close as 25 yards, but you can see that there’s a difference between a level gun and one that’s not level. We’ll get to that in a moment.
What the level does
The scope is mounted above the barrel, so it’s adjusted to look through the trajectory of the pellet so that the point of impact coincides with the aim point at a certain distance from the gun. However, if you tip the gun to either side and then sight it, your crosshairs will still be on the point of aim, but the barrel will no longer be directly below the scope. It will be to one side or the other, depending on which way the rifle leans.
Our intrepid blog reader duskwight was kind enough to give us a link to a superb animation of this phenomenon. You will find it here. Someone (Wulfraed?) said that a gun will describe an arced impact point if the cant is shifted through an arc, left to right. That’s exactly what the online animation shows, and it’s exactly what a scope level does for you.
One reader asked if the scope level would still make a difference if the scope had been optically centered. Yes, it would. There’s no relationship between optically centering a scope and using a scope level. The former simply allows you to adjust the elevation for different ranges without the shot group moving from side to side because the scope stays centered all the time, while the latter relates to how the scope and rifle are actually held when fired. The first is optical, and the second is physical.
The test is straightforward. First, I seasoned the bore with several shots. Then, I fired a group of 10 shots at 25 yards with the rifle canted to the right for 5 shots and to the left for 5 shots. The cant was controlled by the position of the bubble in the level, and I stopped tilting the rifle the moment the bubble came to the end of its travel. Obviously, there’s some error in this, as the bubble level is not a precision instrument, but I think you’ll get the idea.
And here’s the group. Do you see that you cannot tell that the rifle has been purposely canted in two different directions? This just looks like a large group for a Whiscombe at 25 yards. Group measures 0.905 inches between centers.
Next, I shot another group of 10, only this time I completely disregarded the level. I just shot and tried to hold the gun level from the visual cues seen through the scope. This is the same way I shot the rifle during the initial accuracy test.
For this group of ten Kodiaks I disregarded the level. I tried to keep the rifle level by visual cues through the scope, but that was all. Group measures 0.874 inches between centers — or not much better than when I was purposely canting the rifle in two different directions. Also note how much like the first group this one looks.
For the last group I leveled the rifle for each shot. I was also careful to hold the rifle exactly like I was holding it for the other two groups. The results are very telling.
This group of 10 Kodiaks was shot with the rifle leveled each time. It measured 0.624 inches across the centers; however, if the stray ninth shot is omitted, it would measure 0.36 inches. If you check the last accuracy test I did with Kodiaks on Friday, you’ll see that this result is very close.
I’ve seen this same test result repeated numerous times over the years, so I don’t feel the need to run multiple groups and do a sample of each type, but you’re free to do so. I know this is what happens when a scope level is used, which is why I used one when I competed in field target.
You might ask why I don’t always use a level when testing airguns. The short answer: time. It takes a lot longer to settle down and check everything when you shoot this way, and I don’t think it’s always necessary, anymore than I would use a minute-of-angle rifle in a firefight. An AR 15/M16 is fine for that kind of work. But when real accuracy is on the line, a level brings out the very best a rifle has to offer.