Semiautomatics and accuracy
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- Trolling for questions
- Why are semautomatic firearms less accurate?
- Semiautos are accurate!
- Matt’s Garand
- My Garand
- When you fire
- Not even a Garand!
- Why does it matter?
- Airgun accuracy
- Pellets are plastic
- Seating consistency
Today’s topic was suggested by some comments from reader Matt61. He says, “The comparisons at the beginning of the post between firearms and airguns and semiauto vs. bolt-action raise a lot of questions for me. I take it that airgun repeater level accuracy is better than firearm repeater accuracy. Why? If it’s because the round is moved by air instead of a bolt, what difference does that make? The bolt seems like it would be more secure. This all has to do with the mechanism of operation so I guess it really is one question about the difference between semiauto and bolt actions. Once the bolt chambers the round into the chamber, what difference does it make whether the round comes single-shot or from a magazine or whether the bolt is operated by hand or by gunpowder gases (firearm) or air? This history would seem to be erased once the round is in the chamber. So why the differences in semiauto, firearm and airgun accuracy?”
Trolling for questions
I know that many of my readers have questions that go unanswered because they are never asked. So I sometimes put provocative statements into my reports, hoping someone will call me on it. And this time Matt did. Today’s blog should prove even more poignant for him, since he and I spent time together when he first started reloading.
Why are semautomatic firearms less accurate?
The first thing that must be asked is — less accurate than what? The answer is — less accurate than single-shot firearms that have been designed for accuracy. Notice I didn’t say all single-shots. There are a great many single shots that are less accurate than many semiautos. What I am talking about today is the ultimate in accuracy. A different way to view the question is to realize that all world records for pinpoint accuracy belong to single-shots. I’m not talking about hitting a gong at 1,000 yards. I’m talking about putting 10 shots into a group that’s smaller than a small dinner plate at that distance.
Semiautos are accurate!
Okay, I can hear your engines revving up right now! What do I mean by accuracy? A semiauto that can put five shots into a half inch at 100 yards is accurate, isn’t it? Yes, it is. But a precision built benchrest rifle can put ten bullets from the same caliber ammunition into less that one-tenth of an inch at the same distance. Ain’t no semiauto on this planet that can do that!
To understand why this is true we need to consider what Matt has said, “…Once the bolt chambers the round into the chamber, what difference does it make whether the round comes single-shot or from a magazine or whether the bolt is operated by hand or by gunpowder gases …”
If the cartridges were fluid (plastic is the correct term) instead of solid, Matt would have a point. The chamber would be the controlling factor and all cartridges would conform (be squashed) to its internal dimensions. But that cannot and does not happen. A cartridge is rigid and therefore HAS to be smaller in size than the chamber into which it is inserted. Otherwise, you will not be able to get it all the way in! The difference can be very small, and right there is where we get to the difference in accuracy between semiautos and single-shots.
Now, Matt also talked about bolt action firearms. They break down to repeaters and single shots. I’ll talk about both as we go.
To function reliably in semiautomatic actions, the cartridge must be a LOT smaller than the chamber. Matt knows this better than most because he owns a custom-built semiautomatic M1 Garand that was made for target shooting. Its chamber and other mechanical dimensions (headspace being the primary one) are much smaller than off-the-rack Garands. Matt bought his rifle partly because of the Garand’s reputation for reliable operation under adverse conditions. Then he had it customized for match shooting, which did away with that hallmark trait. When he got it back it was very hard to disassemble — which is the complete opposite of the Garand. The gunsmith told him the rifle didn’t need to be taken apart, as long as he kept it clean and lubricated.
Matt’s Garand is extremely fussy about the ammunition it will accept. That’s good from the standpoint of accuracy, but not so good from the standpoint of overall reliability. Matt, tell them!
On the other hand I own what I just referred to as a “rack Garand.” It’s a standard semiautomatic rifle, built from parts made by many different manufacturers to government specifications. In other words, it’s loose! The Garand rifle operates well under adverse conditions precisely because it’s so loose. By that I mean the parts have a lot of room where they need it, to be able to tolerate dirt and foreign material while still functioning.
My Garand is a nice example of that rifle. There is nothing special about it, though the condition is probably 85 percent or better.
My rifle would never win an accuracy contest against Matt’s rifle, so long as he was using ammunition prepared specially for it. My rifle, in contrast, will shoot almost anything you put in it because it so loose. My 3-inch 100-yard groups will not hold a candle to Matt’s 1.5-inch groups, but my rifle will shoot them with almost any kind of ammo.
However, if a 45 lb. single shot bolt action rifle were to be built in .30-06 (the Garand caliber) to benchrest standards, it would easily best Matt’s Garand. That will not happen, because .30-06 is not a benchrest caliber, but the comparison is still valid.
And, if such a benchrest rifle were to be built, it would be extremely particular about its ammunition, because it would have a match-grade chamber. Read that as a chamber that is cut extremely tight! That chamber would still have to be larger than the cartridges that are loaded into it, but the tolerances would be small enough that you would probably notice some resistance as each cartridge was chambered. You might not remember this but my single shot Hammerli model 100 free pistol has a chamber so tight that it will not chamber most .22 long rifle ammunition available today.
Those are the two extremes — tight and loose. All bolt action repeaters are somewhere in-between, with the more accurate of them closer to the tight end. But they cannot be at the extreme tight end because the cartridges have to cycle through their actions before being loaded into the chamber. Is this starting to make sense?
When you fire
When a cartridge fires, the expanding gasses generated by the rapidly-burning gunpowder (it burns so fast it sounds like an explosion) create great pressure inside the gun. The .30-06 we are discussing generates around 50,000 psi. The bullet seals these gasses at the front of the barrel, but it is so light and the pressure is so great that it shoots out of the barrel very fast. The cartridge case seals the gasses at the rear of the barrel, preventing them from going backwards, just as the bullet seals them up front. This is where the gun’s recoil comes from.
To seal the expanding gasses, the case expands to press tightly against the chamber wall. The bolt holds it from going backward, so it can only move to the side like a brass balloon, until it is pressed tightly against the chamber wall.
When the cartridge case is extracted from the chamber it retains most of the expanded dimensions, though not all of them or you wouldn’t be able to extract it. If the case is reloaded for a repeating rifle it must be squeezed back down (sized) to a standard size to feed reliably through the repeating mechanism. If the repeater is operated manually the case tolerances can be closer to the chamber specifications because it will be cycled through the action by hand, where some care can be given to proper feeding. If the repeater is semiautomatic, the tolerances need to be greater because the feeding will be accomplished at high speed by the force of springs.
If the rifle is a single shot, however, the cartridge case tolerances can be very close to the chamber’s dimensions, since it will be hand-fed with great care by a shooter who understands the gun. Cartridges for single shots don’t need to be resized if they are going to go back into the same gun. In fact, they seldom are. Their necks that hold the bullets have to be resized, but the rest of the case can stay just as it is after firing.
Not even a Garand!
If I was to try to shoot reloaded ammo that had not been resized in a Garand, even that great rifle would jam. So, the dimensions of the ammunition have to have enough clearance that the cartridge will enter the chamber fully and allow the bolt to close. Because, unless that happens, the rifle will not fire. It has been designed that way for safety.
Why does it matter?
What does the fit of the cartridge have to do with accuracy? A lot! The alignment of the bullet with the bore is what’s at stake. The more in line the bullet is with the bore on a repeated basis, the more accurate the gun is. Harry Pope shot 10 lead bullets into 0.20-inches at 200 yards once (and only once!) with a rifle that loaded the cartridge from the breech and the bullet from the muzzle. By pushing the bullet from the muzzle down to the breech, Pope ensured as perfect bore alignment time after time. It was a great amount of work for a very small improvement, but Pope guns were built that way and they held world records for years and decades. They even had a name for guns using that method of loading. They called them muzzle-loading breechloaders!
Alignment of the bullet with the bore is the critical element that marksmen chase for ultimate accuracy. And, if you understand what I have tried to explain, you now understand why a semiautomatic can never be as potentially accurate as a single shot, when everything that can be done for each of them is done. And bolt action repeaters fall somewhere in-between. We are talking potential now — remember that. This is why snipers, real snipers and not just mall ninjas who call themselves snipers, still use bolt action rifles.
Matt grouped firearms and airguns together in his question, but they are not the same. They aren’t because pellets don’t normally fit in chambers. There are a few exceptions, but in the majority of cases, pellets start out inside the bore — no chamber involved. So, everything I have carefully explained above that relates to firearms is irrelevant when we talk about pellet gun accuracy.
Pellets are plastic
Pellets are also made from lead that is very malleable. They conform readily to the shape of the bore they are loaded into. To use the term I used when talking about firearm cartridges, pellets are indeed plastic. I’m not saying they are composed of plastic. I’m saying their properties of solidity are such that they can shape themselves to fit a rifled bore. They are plastic in that sense. Plastic can be an adjective as well as a noun, as in — “…capable of being molded or of receiving form.” Plastic like modeling clay!
In Harry Pope’s day (late 19th century) rifle bullets were mostly made from very soft lead alloy. They were plastic, just like our pellets. Today’s most accurate bullets are not made from soft lead. They are made from multiple metals and are held to incredibly tight manufacturing tolerances, rather than relying on the plastic property of lead. That’s all I want to say about that, because I want to focus on pellets.
Once the pellet is in the breech of the gun, it does conform to the rifling in the bore, similar to what Matt said about a cartridge inside a firearm chamber. If the pellet has entered the bore without damage and if the fit of the pellet to the bore is close, the pellet has a good chance of being accurate, as long as the pellet itself is made to very tight tolerances. Each pellet must be identical to all other pellets of the same type.
We have all seen what a wonderful job the PelletGage does. So the importance of pellet fit and consistency, as far as accuracy is concerned, is understood. What that means is use premium pellets for premium accuracy. And get them seated in the bore the same way every time.
So, if a semiautomatic air rifle (or pistol) will seat a pellet the same every time and not damage it in the process, there is a good chance for accuracy. Of course with a bolt action rifle the pellet is seated by the bolt nose and the bolt is operated manually, so the chances for consistency are even greater. That’s because the shooter can pay more attention to how he seats each pellet.
But the single shot air rifle has the best potential of all. If it is a bolt action, the potential is about the same as for a bolt action repeater, because both use a bolt to seat the pellet. In really accurate air rifles extreme care is takes to how consistently the bolt seats the pellet. If the pellet is seated with the greatest care, the potential for consistency is also the greatest.
So, Matt, a semiautomatic firearm has a problem with ammunition. A semiautomatic pellet gun has more of a problem with pellet seating consistency than a manual bolt action pellet gun. That was what I meant by my remark.