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by Tom Gaylord
Copyright PyramydAir.com ©2009. All Rights Reserved.
This article originally appeared in Airgun Revue #2, published in 1998.
Accuracy--it's not a given! Part 2: The projectile
Accuracy--it's not a given! Part 3: The sights
"I want to tune my airgun so that it shoots at least 1,000 f.p.s. How much more accuracy can I expect?"
"If I can get someone to tune my gun so it shoots smoothly, will that make it more accurate?"
"I bought a recoilless airgun for the extra accuracy they are known to have. Why is my friend's recoiling gun able to shoot tighter groups than mine?"
Sound like something you might have said? All of us have said these things at one time or another. What is accuracy, and how do we get it?
Accuracy is several things, all of which must be present for results to pay off. For starters, accuracy is consistency--the repeated striking of a shot in the same place. New shooters are sometimes surprised when a veteran is more pleased with a tight group of shots anywhere on the paper than with a random shot through the center of the bullseye. I can always move a tight group by adjusting the sights, but nothing I do can guarantee the repeat of a lucky shot.
Some guns are "twitchy." They can be made to shoot quite well, but the shooter must stay on top of several factors, such as hold and the ambient weather conditions, to make them perform.
Other guns have been remarkably stable--shooting well in all conditions. They would even suffer abuse and continue to group well. These are the "natural" shooters, and I advise you to keep every one of them you come in contact with, because they aren't that common.
Accuracy is a function of the shooter, as well. The old warning to beware of the man who owns just one gun because he probably knows how to use it--is true.
I'm convinced that most of the accuracy in any gun resides in the barrel. The barrel guides the bullet the whole time it's under the shooter's control, and it's where the bullet gets its physical programming for the rest of its ballistic flight. Good barrel=good accuracy, as long as the right things are done.
In a rifled gun, which is all this article will examine, the rifling affects many of the bullet's physical characteristics (and vice-versa). Among these are rate of twist, bore friction and bullet deformation on both the base and sides. All these things are important, but we'll look just at twist rate. The rate of twist is a measure of how many inches of barrel a bullet must traverse to rotate one time. For example, a barrel that rotates a bullet one complete turn in 10 inches of travel is said to have a one-in-ten-inch twist rate.
Any ballistic projectile can be stabilized by rotating it. A gyroscope provides the proof for this, even though most gyroscopes we are familiar with are not ballistic projectiles. But bullets are! They rotate on their long axis like American footballs thrown in a forward pass. And, like footballs, they travel farther and straighter if they're thrown with a spiral twist. A spear also travels straighter if it's spiraled when thrown. So does an arrow.
If the twist is fast enough, the bullet will be stabilized well and travel very straight. If the twist is slower, the stability will not be as good, and the bullet may begin to go in an erratic direction. In Vietnam, the early US M16 rifles were said to be more deadly than the larger-caliber rifles they replaced because their smaller-caliber bullets naturally tumbled when they hit flesh. Actually, the early M16s had an incorrect twist rate for the bullets they fired, and the striking of flesh simply made this more apparent. Left alone, they destabilized in flight and became erratic at long distances. If they were so great as flesh destroyers, why did the Army bother to change the twist rate in later rifles? The answer is accuracy.
Unfortunately, the twist can also be made too fast. That will put undue stress on both the bullet and the barrel. In firearms, barrels with a fast twist tend to erode more quickly. In airguns, they don't erode, but they may begin to strip lead from the pellet, which can build up in the bore of the gun. Since most airgunners don't clean their bores, this could cause a problem.
I'm not aware of anything that has been published on twist rates for airguns. There has been a lot of work done on firearm twist rates, especially in the blackpowder field, but nothing on airguns. I'm quite sure such work has been done by the individual manufacturers (probably several times, since no one publishes it) but that's of no help to airgunners.
Before this discussion proceeds, I must mention that I'm NOT going to tell you what the "right" twist rate is for an airgun, because I don't know. What I DO know is that once a bullet or pellet leaves the muzzle of the gun, twist rate ceases, because the bullet is no longer under its direct influence. Instead, the twist of the barrel has imparted a spin to the bullet that's now expressed in rotations per second (r.p.s.). Like any mechanical force, r.p.s. will eventually cease; but the flight of a bullet is so brief, in comparison to the length of time it takes to stop rotating, that it's irrelevant.
The thing to remember about r.p.s. is that it increases as velocity increases. It's a direct relationship. So, a pellet that's unstable at slow speeds may stabilize when it goes faster. Think of a pellet shot from a one-in-ten-inch-twist barrel. If it exits the bore at 1,000 f.p.s., it will also be spinning (rotating on its axis) at a rate of 1,200 r.p.s., because there are 12,000 inches in 1,000 feet, and the pellet is rotating once every 10 inches of travel while it's inside the barrel. It won't change much from that rotational speed after it leaves the barrel, because there's very little besides air friction to slow it down.
On the other hand, the VELOCITY of the pellet begins to drop immediately after it leaves the muzzle--and with some guns and states of tune, even before. So, the pellet remains at 1,200 r.p.s. for essentially its entire flight. In reality, it does slow just a bit, but the amount is so small it doesn't affect anything we're looking at here.
The same pellet shot from a gun at 800 f.p.s. would be rotating 960 r.p.s. (800 x 12 = 9600 ÷ by 10 = 960). At 600 f.p.s., the r.p.s. would be 720. Remembering that lighter pellets require a slower spin to be stabilized than heavier pellets, they can be expected to do better at slow muzzle velocities. Downrange velocity doesn't mean very much for this computation. Longer pellets need more spin to stabilize them than do shorter ones. So, for any rifled barrel, there will be pellets that perform better because the twist is fast enough for them and for pellets that are unstable because of their size.
The compromise is this: if a barrel can stabilize a heavier pellet, it will also over-stabilize a lighter pellet. Over-stabilization isn't bad--it's just an expression that means there's a greater spin than required to stabilize the pellet in question. It will still be accurate, unless it begins to strip in the bore, as described earlier.
The twist rate of the Korean-made Career 707 rifle is extremely fast, at one turn in eight inches. Although it's no longer imported to the U.S., it had a reputation as one of the most accurate air rifles on the market. It shoots an extra-heavy, extra-long 29.6-grain domed pellet that can group five shots in 0.30" at 50 yards. That's only the beginning, though, as this air rifle is as accurate as many .22 rimfire rifles clear out to an amazing 100 yards. Most of this is due to the barrel, and most of that is due to the twist rate.
Uniformity is another factor in barrels. The width of the bore (the distance between the opposing walls of the bore) should be as uniform as possible, so the pellet is guided through without undue influence. You might think that this uniformity is a given, but it isn't. It represents one of the most challenging control factors barrel manufacturers have been dealing with for more than a century. Airgun manufacturers sometimes compromise the best work of the barrel makers when they press-fit their barrels into a base block of steel to form the barrel assembly. This operation can swage constrictions into the finest barrels, ruining their near-perfect uniformity.
Another uniformity factor is the height of the rifling lands. If they're not uniform, they'll either impart uneven friction on the pellet, or else they'll fail to grab it at all--allowing it to traverse the bore without guidance. Either condition promotes inaccuracy.
Airgun barrel makers must keep the height of the rifling to the minimum possible, because it robs the pellet of velocity through friction. A lack of uniformity is quickly compounded. This is one of the major problems the Chinese airgun manufacturers face today. They haven't yet gotten a handle on making uniform barrels in the quantities needed to keep pace with the rest of their production. Their fine Olympic models are proof that they can make good barrels when they must, but the average Chinese airgun barrel has most of the uniformity faults mentioned here.
Still another barrel factor is the straightness of the bore. Boring a straight hole is a problem that barrel makers have never solved. There have been great strides in dealing with this problem, though. That's why accurate barrels can be made. But don't assume that any barrel is straight on the inside--no matter how it measures on the outside.
Top barrel makers sometimes bore their barrels in the vertical plane to eliminate the influence of gravity pulling on the boring bit. Another trick is to bore the hole undersized, then ream it to the final dimension.
Even if a hole runs straight, there's no guarantee that it's parallel to the outside of the barrel. I've seen many, even most, firearm barrels off-center this way when they were cut off behind the muzzle. Finishing at the muzzle hides this from the consumer, but it's there just the same. While the barrel can be turned on a lathe after the bore has been drilled and reamed, this operation can also put stresses in the steel that leads to things like uneven movement when the barrel heats during shooting. So, there are no easy answers to the straight-hole/parallel-hole dilemma.
Smoothness inside the bore also affects accuracy. The walls can be uniform and parallel but tiny ridges in the rifling itself can wreak havoc with accuracy. Fortunately, something can be done about this condition. The barrel can be lapped. Lapping is a process that reduces the high spots in metal surfaces to a uniform level. It is done with abrasives and requires skill, or the barrel can be ruined beyond hope of redemption. There are several different methods of lapping a barrel, but they all seek the same result--a smooth, uniform bore.
Don't automatically assume that all barrels NEED to be lapped, though. While all of them have the ridges mentioned, many shoot quite accurately and would only suffer if they were lapped incorrectly. In the 1960s, there was a barrel-lapping fury that swept the firearms industry and ruined many fine bores. In the '90s, the fad came back, with "fire-lapping" being hotly debated in the magazines.
It's probably best to shoot a gun before determining that it needs to be lapped. Only lap the poor performers--leave the good ones alone. And, if you don't have the experience to do the job, hire someone who does.
Barrel vibration is still another factor that greatly affects accuracy. Until recently, vibration was either ignored by manufacturers and shooters, alike, or else it was treated in such a manner as to be rendered neutral. The neutralizing of vibration became embedded into the fabric of good marksmanship, where it resides today in the form of consistency of hold. Top shooters practice it; average ones do not. It's often one of the deciding factors in a match.
For the airgunner, barrel (and even whole gun) vibration is a key to accurate shooting to a much greater extent than for firearms shooters. That's because the pellet is so much slower that it's influenced by the gun to a much greater extent than a bullet. Some airgunners have taken to shortening their barrels in an attempt to get the pellet out before there can be much influence from vibration; but all that does is reduce velocity--and, in spring guns, increase the harshness of the firing cycle.
A better approach to vibration is to cancel it from the shooting equation. I do this by holding my airguns the same way every time. The payoff in spring guns is enormous and dramatic. With gas guns it's much less, but I do it just the same to remain consistent.
I rest the forearm of a spring rifle on the flat of my open palm and grasp the pistol grip very lightly with my shooting hand. I don't pull the rifle into my shoulder. This sets the gun up to recoil and vibrate to the maximum extent it can. That's what I'm after. I let it buck and vibrate all it wants with every shot. That way, when the pellet exits the muzzle, the gun will always be in the same position. That has the effect of always launching the pellet from the same point. If it is properly stabilized, it will fly as true as possible, and I'll be rewarded with the tightest group that gun can make with that pellet.
I figured this out after becoming frustrated with a Beeman C1 carbine a few years back. I was following Beeman's recommendation (in their catalog) to hold the rifle firmly when I shot it. No matter what I did, that C1 just would not group. Finally, in frustration, I laid the rifle on a comforter to see just how poorly it would perform. Lo and behold, I shot a 0.10" five-shot group at 10 meters that way.
This was followed by lots of experimentation with many airguns and observations of others who are known for their prowess. Also, I reflected on my military training with artillery pieces and mortars. They recoil severely when shot, yet they're also highly accurate at many miles distance. The one common denominator seemed to be that they were allowing the barrel group to move as much as it wanted, but ensuring by design that it always started out from the same place. As a result of both my experimentation and observation, I named the hold described above the "artillery hold." I didn't invent the thing--only gave a name to what the good shooters were already doing.
If you agree that my reasoning about the barrel makes sense, why not incorporate it into your quest for accuracy with airguns? There are many other subjects to be dealt with, but the barrel is where it all begins.
Finally, a few years ago, the Browning corporation introduced their BOSS muzzlebrake as a solution to barrel vibration. The BOSS has a movable weight that lets shooters adjust the vibration patterns of the barrel to suit a specific load. This concept is so demonstrable and works so well that Browning held annual competitions for BOSS owners.
Airgun makers are just starting to take barrel vibration into account. John Whiscombe was the first manufacturer to install devices on the barrels of guns he makes, but there are now several other designs in the works, and airgunners are starting to experiment with firearms add-on muzzlebrakes with adjustments. It's only a question of time until you'll be able to buy many airguns with vibration adjustments built in.