Revisiting Gain Twist, Part Two

Let’s fast forward from Pope’s barrel making era about a century ago to the "modern" rifling methods. Most hunting rifle barrels and a fair number of military and custom barrels are rifled using the button method. As you know, the button method does not cut the grooves out but simply displaces metal in a swaging process. Button rifling is much faster to accomplish and that helps reduce costs as compared to cut rifling methods.

One of the criticisms of the button method, however, alleges that the twist rates of some barrels can vary just a little, adversely impacting on accuracy. While the average hunter trying to bag a deer or a Soldier shooting similar sized two legged targets, both at similar modest distances, might not notice any accuracy degradation, some high end snipers and all competition shooters involved in Bench Rest, High Power and all the long range disciplines have greater accuracy standards.

As the legend was told to me, someone made up a "twist rate checker" of some sort. Button rifled barrels were subjected to testing and some were found to vary a little in twist rate A high correlation was found between accuracy and how the twist rate varied. Remember, these are uniform twist barrels that are supposed to have a uniform twist rate from back to front and not gain twists.

Allegedly, barrels whose twist rate became a little faster toward the muzzle were more accurate than ones whose twist rate became slower from back to front (really undesirable) or barrels whose twist rate didn’t vary at all. Bartlein Barrels makes their barrels by the cut rifling method, not the button method. Frank Green at the Bartlein shop tells me that a fair number of the early orders for gain twists came from Bench Rest and Palma competitors who had heard of the twist rate study.

The bench rest guys are ordering barrels with the most modest rates of gain just to capitalize on the potential for improved accuracy. Remember, my AR barrels are going from 1:13" at the back to 1:6.5" at the muzzle, doubling in less than 20" of rifling. The Match Rifle guys, whose barrels are probably longer, are only going from 1:12.25" at the breech to about 1:12.00" at the muzzle, only a quarter inch gain in twist. We are seeing two different camps at work here.

My Service Rifle camp is looking at a more aggressive gain to reduce pressure and soften the obduration of the rear of the bullet. The Bench Rest and Palma shooters are hoping to harvest a little more accuracy and are less worried about pressure or bullet obduration. Military Applications We have all seen pictures of Gatling Guns used around the time of the Civil War. These were multiple barreled guns firing a high volume of fire for the time.

Today’s military has multibarreled weapons in some aircraft that bear quite a resemblance to the old Gatlings, usually chambered in 20 or 30mm calibers and firing much higher volumes. Many of them are used as tank killers. The specifications for the guns state that the rifling must be "progressive". I have not been able to determine the rate of gain used in the rifling of these guns, however, barrel heating in guns such extremely high rates of fire is a major concern.

I have had a number of conventional slow twist 1:12", 1:13", and 1:14" barrels used for shooting 55 grain military ball ammo in my ARs. Accuracy was much better in such barrels than with military barrels having 1:7" twist rates. I also noticed that barrel heating was much less with the slower twist barrels.

By starting the bullet spinning at a modest rate at the back of the barrel and then speeding it up at the muzzle I suspect that the military, in part, may be trying to deal with barrel heating in their rotary cannons. Gain twist is also said to work well with projectiles that have "driving bands" such as ammo fired in the cannons.

Barrel heating can be a real concern in some competitive shooting events such as the Infantry Trophy Team Match ("Rattle Battle") where shooters routinely fire a full 30 round magazine from an AR-15 in under 50 seconds. I would think that the right gain twist, something in the neighborhood of 1:16" x 1:8", would be optimal in reducing barrel heat. Hot barrels can trick the shooter’s vision by excessive barrel mirage. Hot barrels can also cause groups to shift location and get fatter.

Not long after the American Civil War, the Italian military adopted the Carcano rifle which had again twist barrel. Smith & Wesson’s Views Gain twist has seldom been used in factory guns. A few years ago S&W broke with that tradition and introduced the .460 XVR Magnum Revolver, a heavy hunting revolver with a factory gain twist barrel that starts at 1:20" at the back and finishes up at 1:10" twist at the muzzle. That is an aggressive gain.

Like my AR barrels, the pistol doubles its twist but in a shorter (just a shade over 8") distance. The stated reason that S&W gave for use of the gain twist barrel had to do with what they call "bullet skid." They believe that any bullet entering a "fast twist" barrel won’t imme-

diately begin to rotate. Instead, they allege, the bullet will skid along the lands for a little way before the lands engrave into the bullet (either lead or jacketed) and begin to rotate it. As the bullet is skidding it becomes deformed resulting in degraded accuracy. Note that this S&W revolver has a terminal 1:10" twist at the muzzle so the manufacturer has determined that 1:10" is the optimal twist rate to stabilize the bullets anticipated to be shot in the gun.

If S&W had determined that bullets will skid in conventional barrels of that modest twist we can only imagine how much worse rifle bullets accelerating more rapidly will skid in faster twist barrels like 1:8" or 1:6.5" twist used by Service Rifle shooters. More skid, more deformation. Rotational Torque A couple of factors disturb the lay of the firearm as the bullet is traveling down the bore. The first is recoil and we are all familiar with that. It is a reaction to accelerating the bullet forward.

Generally speaking heavier bullets result in more recoil and accelerating them faster exacerbates recoil even further. Rotational torque, on the other hand, is a response to rotating the bullet. The firearm wants to "unscrew" in the opposite direction that the bullet is being forced to rotate. Right hand twists make the bullet rotate to the right and the firearm reacts to the left. Rotational torque is affected by twist rate, velocity, and bullet diameter.

Fatter bullets have more circumference and more surface area. So they resist rotation more than skinny bullets. That means that, all other factors being equal, guns like the .45 ACP will have more rotational torque than guns shooting a skinnier bullet with less bearing surface area like a 9mm. Again, keeping all other factors equal, slow twist barrels result in less rotational torque than faster twist barrels.

If you look at pictures or watch a one handed .45 hard ball National Match bullseye shooter in recoil you will notice that the pistol not only goes way up and off target but it also twists to the side, often by about 45 degrees. That twisting to the side is caused by rotational torque and to get back on target to fire the next shot the torque as well as the recoil have to both be overcome. If gain twist eliminates some of that torque then total recovery should be faster.

Uniform Gas Pressures In a conventionally rifled barrel, chamber pressures are high because we are trying to move the bullet forward while simultaneously getting it spinning. There is great inertial resistance to both. As the bullet moves forward the expanding gas fills the area formerly occupied by the bullet. Eventually, as this process continues and the bullet gets far enough forward, pressures begin to subside a little. Gain twist does the opposite. Pressures still want to drop off as the bullet moves forward.

However, as the bullet does so it keeps running into more and more resistance rotationally because the twist rate keeps going up and up the further the bullet moves toward the muzzle. Gain twist barrels cause less initial pressure at the chamber but as the bullets move forward some of the resistance to the higher rotation works against the forward movement, which by itself would decrease pressure. Throughout the entire travel from the chamber to the muzzle, pressures are more consistent.

That may result in better velocity standard deviations and less extreme velocity spreads. While I am getting good numbers in both categories with my gain twists, I can’t compare them to conventional rifling because I am no longer using any barrels other than gain twists. Barrel Life And Ease Of Cleaning Gain twist has the potential to improve barrel longevity. Bullets get moving sooner. Chamber pressures decrease.

This means that the hot cutting gases have less time to attack the back of the barrel which is the wear out area. Gain twist barrels operate cooler which should result in less fire cracking at the back of the barrel. As a former military armorer I have found that barrel wear out is extremely variable and difficult to predict, however. Barrel life was one of the stated reasons that the Italians adopted the gain twist in their Carcanos. I think accuracy should always be the deciding factor when evaluating barrels.

Like one of my shooters once said to me, "Why would I want a poor shooting firearm to last along time?" I am not seeing the usual severity of bore fouling just forward of the chambers in my gain twist barrels. Bores still need cleaning but easier cleaning equates to less potential to damage the bore, which is always a good thing.

The Future Of Gain Twist I mentioned at the beginning of the article that gain twist has been on again and off again for over a century and a quarter that I am aware of, perhaps even longer. Today we have some really premium barrel makers turning out gain twists and I believe we better understand the improvements they can offer. My guess would be that gain twist will be around for quite awhile into the future.

Now that you know the advantages you may want to try one if again twist fits into the goals of one of your upcoming projects. Sources For Gain Twist Barrels: • Schuemann Barrels, schuemann.com) Colerain Barrel Co., • • Bartlein Barrels, • RKS Enterprises, Box 635 Wimbor ne, AB TOM2GO G