They have the wrong twist rate!: Part 1
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- Then came big bores
- What am I talking about?
- Bullets don’t work like pellets
- Bullet length
- Why bullet diameters matter
- Hard cast lead bullets
- Putting all of this together
- Last story
Before we begin I have some startling news from Umarex USA. Sales manager, Justin Biddle contacted me and told me the new Hammer big bore is regulated.! I certainly did not know that, because this is huge news. It means that those three shots it gets on a fill can all be at the same power level. I have adjusted the SHOT Show report (Part 6) to reflect this new information. And by a strange coincidence, this dovetails nicely with today’s report.
For many years I have written about black powder firearms in this blog, and I have included things like rifling twist rates in those articles. And for years people have written me comments that they appreciate a look at something different, but they would never consider shooting black powder arms or even modern firearms, themselves.
Then came big bores
A few years ago, big bore airguns began rising in popularity. First it was the exotic guns (I call them boutique guns, though Quackenbush, alone, has reached low-rate production levels) from makers like Dennis Quackenbush that a few lucky shooters were fortunate enough to acquire. Before long, though, the Koreans began making big bores airguns in volumes large enough that U.S. shooters were able to buy big bore airguns at will.
Dennis makes his rifles for shooters who understand the ballistics and needs of a big bore airgun. They either come from a background of black powder or they embraced it quickly after acquiring their airguns.
In sharp contrast, the Korean big bore makers fumbled the ball more than once — not understanding the subtleties of the variations in actual bore diameter among “common” calibers like .45 (there is .451/.452 for the .45 ACP cartridge, .454 for the modern .45 Colt caliber, .457 for the antique .45 Colt caliber — same cartridge, but the bore diameter changed in the early part of the 20th century, and .457/.458/.459 for the .45 rifle cartridges like the .45-70 and others. Each foreign manufacturer produced rifles with barrels having whatever bore diameter they wanted, and for a long while there was chaos in the world of big bore airgun.
They followed that with a slew of smaller rifles they insisted were 9mm, which is a bore diameter of .355/.356. I suppose they thought that the 9mm cartridge is so ubiquitous that shooters would be able to get bullets to shoot anywhere. That is true, but 9mm handgun cartridges almost always use jacketed bullets that don’t work in air rifles. Air rifles like soft lead bullets, and there are very few of those in 9mm. But there are a host of them in .357. Heck — that’s only one-thousandth of an inch larger, the reasoning probably went. Those should work in these guns, too, shouldn’t they?
What am I talking about?
You probably think I am ranting about bore diameters today, but I’m not. I’m just getting warmed up to talk about twist rates. However, until you understand the dynamics of everything involved, none of this bullet stuff will make sense. That is why I wrote all those articles about black powder firearms in an airgun blog.
Bullets don’t work like pellets
I am a real fuddy-duddy about calling a bullet a bullet! Bullets are not diabolo pellets, and calling them that on the package doesn’t change their dynamics. Bullets have to be stabilized by spin, because they don’t have the same high drag factors that diabolo pellets have to keep them pointed straight ahead while in flight.
Spin is induced by rifling, which are lands or ridges cut in a spiral inside the bore. These ridges or lands cut into the sides of the bullet (it’s called engraving) and make it turn to follow them. As the bullet travels down the barrel it rotates according to the rate at which the lands turn. And here is the first truth you need to grasp. Whether the barrel is 2 inches long or twenty inches long, if the twist rate is the same and the velocity remains identical, the bullet will always spin at the same rate. That’s right — a 2-inch barrel with a 1:10” twist spins a bullet just as fast as a 20-inch barrel with the same twist — as long as the bullet is driven through both barrels at the same velocity. Longer barrels don’t spin the bullet more.
And, here is the second truth you need to learn — regardless of the twist rate, the faster the bullet travels through a barrel, the faster it spins. It is the bullet’s velocity that determines how fast it spins, not the barrel’s twist rate. That said, a faster twist barrel will spin a bullet faster at a given velocity.
So, if the barrel has a twist rate of one turn for every 12 inches of travel (written as 1:12”) and the bullet is shot at 1,000 f.p.s. it exits the muzzle spinning 1,000 times per second. If it is shot through the same bore at 2000 f.p.s. it exits the muzzle spinning 2,000 times per second. The length of the barrel has no bearing on this.
The third truth about bullet spin is it does not slow down very fast after leaving the barrel. When a bullet finally falls to earth after hundreds or even thousands of yards of travel, it is still spinning nearly as fast as when it exited the muzzle.
The longer the bullet (in relation to its diameter), the faster it needs to spin to stabilize. Those who reload understand this very well, because their favorite bullets are only accurate within a narrow band of muzzle velocities.
The longer a lead bullet of a given diameter is, the heavier it is and the slower a big bore air rifle will drive it. Less muzzle velocity equals less spin, as we have learned. But longer bullets need more spin to stabilize. We have a dilemma. Firearm shooters resolve this by using gun powders with different burning rates that generate greater pressures for more push. Black powder shooters can only load more powder and shoot through longer barrels, both of which soon reach the point of diminishing returns. Big bore air rifle shooters can only use the longest barrels they can get, and sometimes they can increase the reservoir pressure, though that isn’t as straightforward as it sounds. In other words, big bore airguners need to shoot the right bullets to get the best performance. They don’t have the flexibility of bullet selection that a firearm shooter has.
Why bullet diameters matter
And this is where the bullet diameter matters. Let’s consider a .45 caliber bullet, for example. A .45 ACP bullet measures .451/.452-inches in diameter and it is very short. The weight range for this bullet in lead stretches from a low of around 160 grains to a high around 230 grains. The lighter bullet is shorter, so it needs to spin less fast to stabilize. That means you can shoot it slower from a gun with a given twist rate barrel, or you can shoot it through a barrel whose twist rate is slower than a heavier bullet would require.
A given big bore will drive this pistol bullet (.45 ACP) at a certain velocity. The shooter needs to find the weight and length bullet that stabilizes the best and is also the most accurate in his rifle. When he does — that’s it!!! You can’t make a 200 foot-pound Korean big bore shoot a 500-grain lead slug, even if you could find one in .451/.452, which you never will.
Some big bores have a limited range of power adjustment or they have two fixed power levels. Even with that flexibility they will still only shoot best with a narrow range of soft lead bullets (both diameter and length). As much as you may want to shoot something different, it “ain’t a’ gonna” happen. Shooters who are familiar with black powder arms understand this, because that’s been their experience. But a lot of airgunners are getting into big bores today and have missed all of this discussion about bullets, so they get surprised. I consider it my job to inform them, but I only reach the ones who will read and understand.
Hard cast lead bullets
As if the aforementioned discussion wasn’t confusing enough, there is another fly in the ointment — hard cast lead bullets. They exist for one purpose, and one purpose only — to allow heavy charges of powder to be used with lead bullets. In other words, to drive lead bullets faster than they are normally designed to be driven. Hard cast lead bullets are hardened through alloying antimony with oure lead that makes the lead much more resistant to deformation. Unfortunately, besides keeping the bullet from “skidding” in the rifling under extreme pressures, this process doesn’t add anything else that is desirable. It causes massive lead deposits to be left in the bore that soft lead bullets will not leave unless they are driven way too fast — far faster than any big bore airgun can drive them.
But many bullets on the market are hard cast. They look better, deform less through handling, fill out the mold better and they are much easier and quicker to cast. So bullet makers tend to make more of them and shooters tend to buy and shoot them without realizing what they are doing. They end up with lower velocities, bullets that don’t deform well in game (sometimes they break apart without mushrooming) and barrels that lead up with just 40-50 bullets shot through them instead of the hundreds of shots they could get with soft lead.
Putting all of this together
If you intend getting into big bore airguns, or if you are already there and things aren’t going as well as you had hoped, your bullets may be the reason. Nobody in their right mind would buy a Corvette automobile to tow a horse trailer, but big bore airgunners are doing the equivalent when they select bullets that are not compatible with their airguns.
If all of this discussion sounds real deep, consider this. If you and I were to have a conversation for one hour on the same subject I would probably tell you five times as much about bullets and still just scratch the surface. Don’t be that shooter who buys a big bore air rifle on a whim and then whines because it isn’t what you thought it should be. Learn about this part of the hobby and enjoy yourself! That is why I am making this report a Part 1.
Today’s title is taken from a lot of discussions on the airgun forums by shooters who have bought big bores but never invested the time to discover for what purpose they were designed. They are too ready to condem before understanding.
I remember talking to a guy years ago who owned a vintage Remington Rolling Block rifle in .43 Spanish caliber. That is the military designation for the .44-77 civilian caliber cartridge (.44 caliber and 77 grains of black powder in the cartridge0>. The guy went out and bought some reloads, then complained to me that his rifle couldn’t group 5 shots in less than two feet at 100 yards.
A couple years later I acquired a vintage Argentine rolling block in the same caliber, but I researched the gun, and the caliber. I found out what the proper loads were. I bought a bullet mold and cast bullets of the correct size and lead softness for it. Eventually I shot 5-shot groups of perhaps 5 inches at the same 100 yards with my rifle. My .43 Spanish rifle had a bore diameter of .4385-inches and favored a .439 soft lead bullet weighing 375 grains and traveling 1300 f.p.s. or more.
Isn’t that better than just buying an expensive gun, loading it with whatever ammo you can get and then whining because the gun isn’t accurate?
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