Revisiting Gain Twist, Part One

Some shooters are under the impression that gain twist barrels (sometimes called "progressive rifling") area recent invention. They have been around longer than many of us are aware. One of the most famous barrel makers of all times was Harry Pope and he was making gain twists back in the late 1800’sand early 1900’s. Writings from the period say that his gain twist barrels were unsurpassed for accuracy.

How much of that tack driving ability was due to the gain twist configuration versus good tooling and a sound work ethic, no one can say. Given the period in history that Pope was making his gain twist barrels, it is likely they were used a lot for shooting lead bullets. The popularity of gain twist barrels has been like a roller coaster ever since. As jacketed bullets became more popular, there may have been some confusion as to whether gain twist barrels would work with them.

Even to this day, I still hear some people questioning if gain twist is just for lead bullets. Today, gain twist barrels are available for muzzle loaders employing lead projectiles but you can also purchase them for several brands of pistols which will shoot either lead or jacketed bullets and there are US and Canadians firms making gain twist rifle barrels for jacketed bullets. What It Is The name "gain twist" hints at how it works.

The rifling spins the bullet slow at the back of the barrel and increases the twist rate incrementally closer to the muzzle. As an example, the gain twist barrels I am using in my 20" AR-15 service rifle barrels go from 1:13" twist just at the front of the chamber to 1:6.5" twist as the bullet exits the muzzle. In experimenting, I had one of these barrels split lengthwise.

I took pictures to show the change in twist from back to front, however, because the lands are just a little more than the thickness of a sheet of paper taller than the grooves my limited photography skills just couldn’t capture this well enough. If you’d like to see this barrel, stop in and visit me on Commercial Row at Camp Perry, OH in late July and early August during the CMP Service Rifle National Matches. We all know that longer bullets require a faster spin to stabilize than shorter bullets.

The 1:13" twist at the back of my AR’s will nicely stabilize short 55 grain military style bullets. A uniform 1:13" twist barrel doesn’t spin the long 80 and 90 grain bullets that Service Rifle shooters use for 600 and 1,000 yard competitions fast enough. The 90 grain bullets require a fast 1:6.5" twist to stabilize them. With regard to stabilizing the bullet, the only twist that the bullet "cares about" is the final one at the muzzle.

So 90 grain bullets are just as stable in flight with again twist that starts spinning them very slowly at first and ramps up the spin at the muzzle as they would be if spun uniformly real fast all the way from the front of the chamber to the muzzle. So why bother?

I just said that 90 grain bullets shot from a conventional barrel that has the rifling cut at 1:6.5" twist all the way from just in front of the chamber to the muzzle will stabilize bullets equal to again twist that begins spinning the bullet at the slower 1:13" twist at the back and increases the rate of spin to 1:6.5" at the muzzle. So why pay a barrel maker a few dollars more for the gain twist when the standard 1:6.5" twist will do just as well at stabilizing the bullet?

The answer, simply put, is that stabilizing the bullet is not the only consideration. As near as I have been able to trace back, the first 90 grain bullet for the AR-15 was designed by Jimmy Knox back around 2004. That bullet offered by his firm, JLK (his initials), was a very low drag design with an impressive ballistic coefficient.

The Army Marksmanship Unit was looking for a good 1,000 yard service rifle alternative to 80 grain boat tails that are too susceptible to wind drift and just make it to the target supersonic at that range. They jumped on the JLK and, considering the very limited selection of conventional rifled 1:6.5" twist barrels available at the time, they did quite well.

One of their shooters, using JLK 90’s, broke an Interservice 1,000 yard service rifle record that I had previously set with an M14 shooting heavy .308 bullets that had held for nearly a decade.

Shooting VLD bullets is a pain in the butt for anyone. First, you have to shoot test groups to determine the amount of bullet jam each gun wants. The bullet is seated a little longer than the chamber and is actually driven up into the throat when the bolt closes. Each gun has its own bullet seating "sweet spot" where accuracy is best. In my experience with JLK 90’s, this jam can run anywhere from 0.001" to 0.010" but is always gun specific.

Then, as the throat erodes forward from being shot, the shooter must continually gauge the new length and readjust his bullet seating die to compensate. Bullets get set out longer so the amount of jam remains in the sweet spot originally identified for the individual rifle. This process is generally referred to as "chasing the throat." Eventually, as chambers get longer and longer, one may even have to increase the powder charge in order to maintain the original muzzle velocity.

There is nothing particularly difficult with all of this but it is time consuming and requires diligent attention to detail and record keeping. It is a fair amount of effort for one gun. For a military rifle team it becomes a mountain of work! The AMU used their support staff of one or two "smart guys" to chase the throats for the whole squad. Those poor guys must have had very little time for anything else.

As Dirty Harry famously said, "A man ought to know his own limitations." So the AMU limited its 90 grain VLD work to just 1,000 yard long range guns and I don’t think they dabbled much with them for 600 yard shooting. The respectable B. C. of the JLK bullet would have provided a fair amount of 600 yard "wind proofing" but at the cost of monumental volumes of work. Boat tail bullets are normally jumped as opposed to being jammed. That is, they are seated just a little shorter than the chamber.

Boat tails are much less maintenance to keep up with as there is no requirement to chase the throat. Most military rifle "For what it is worth, they still credit me as being the first reliable source to experience bullet blow ups with the 90’s. Since then a whole lot of shooters have shared the experience, however.

Sierra concluded that the jacket could have stood to be a little thicker." teams use boat tails but there were no 90 grain boat tails on the market back in 2004 when the AMU was first interested, so they made due with the high maintenance VLD’s while probably hoping that some bullet company would introduce a suitable boat tail design. Sierra did just that in 2005. Supplies of Sierra’s new 90 grain boat tail were a little difficult to come by at first.

On top of that, Jimmy Knox ended up selling his company eventually to Swampworks, Inc (swampworks.com). I was the Armorer for the Army Reserve Service Rifle Team during all this and was watching the goings on and also hoping for a low maintenance 90 grain boat tail to hit the market. When I was able to get my hands on a supply of the Sierras in the early spring of 2006 I began load development work using across section of the only barrels that were available at the time.

A true 6.5" twist, four groove from Krieger, a 6.5" twist three groove from Pac Nor and a straight 6" twist, six groove from Douglas. I consulted with Sierra and they recommended hitting a muzzle velocity of 2,650 feet per second to keep bullets supersonic and strong out to 1,000 yards. There was (and still is) very little load data available for 90 grain bullets in the .223 Remington. In order to achieve the 2,650 fps. Sierra recommended in my 20" barrels, I had to go way over published maximum loads.

I was using new military brass with crimped in primers and was pleasantly surprised to be able to achieve Sierra’s recommended velocity with no primer retention problems in my Wylde chambers. Accuracy was good. Dependability, not so much. As my speeds began to approach the recommended 2,650 fps. I began to experience mid air bullet blow ups in all three barrels. The Kriegers with their customarily tight 0.218" bores really ate the Sierras for lunch.

The odd numbered three groove Pac Nor was the most "Sierra friendly" but still blew up some bullets. Of course, I contacted Sierra and shared my experiences them. For what it is worth, they still credit me as being the first reliable source to experience bullet blow ups with the 90’s. Since then a whole lot of shooters have shared the experience, however.

Sierra concluded that the jacket could have stood to be a little thicker and felt that rough obduration of the back of the bullet was the major cause of the midair bullet failures. The AMU was aware of my bullet blow ups. I guess they concluded that with the barrels that were currently on the market there was no way to achieve the target 2,650 fps speed without the danger of blowing up the Sierras. I believe they continued to use up what JLK’s they had and later abandoned their 90 grain project.

A couple of years later they came out with AR-10’sand successfully petitioned the NRA to accept it as a legal Service Rifle. While they have done well with the new .308’sat 1,000 yards, I don’t believe they use them for regular 200 to 600 yard Across The Course shooting, likely due to the heavy recoil trade off in the rapid fires. The Army Reserve stuck with the 90’sa little longer. At about $3,000 a pop for a custom AR-10, the price tag for the Reserves was always going to be a hurtle we couldn’t over-

come. The Reserves didn’t have the deep financial pockets of the AMU. I also felt that if we could get the 90 grain Sierras to quit blowing up they would give usa decided advantage at 600 yards without the recoil penalty of going to a .308. We had given up the old M14’s years ago both because of excessive recoil in rapid fires and nagging maintenance issues. Nobody in their right mind wanted to go back to .30 caliber even for 1,000 yard shooting, much less Across The Course work.

There was also no question that staying with the AR-15 platform and just rebarreling the uppers to 6.5 twist would have a significant ergonomic advantage. While the AR-10 is a Stoner platform, it is not an exact duplicate of the AR-15 and there are dimensional differences. The AR-10 is much more similar to the AR-15 than an M14, which is completely different ergonomically. If you are the AMU and have five training days a week you can afford to spend one day shooting the AR-10.

Reservists and civilians don’t have that training luxury. So, I decided to work with the AR barrel manufacturers in hopes that we could come up with a "Sierra friendly" configuration. I had tried two gain twist barrels for controlling pressure earlier in my shooting career with the Reserves and I suspected that the slow beginning twist at the back of the barrel might help with the rough obduration that Sierra felt was the problem.

I canvassed the barrel market but couldn’t find a single source for gain twists in the United States. One of the companies I had purchased gain twist barrels from a few years prior had gone out of business and the other had discontinued gain twists altogether. I sent inquiries to England, Australia and Tim-Buck-Two but couldn’t find anyone making gain twist rifle barrels. So I tried other avenues in my attempt to find a "Sierra friendly" barrel.

I found most of the barrel makers to be very "Using my gain twist 1:13" x 1:6.5" configuration and the decreased pressure that results, I can dump about ½ grain more powder into my cartridges which brings my velocity to the target 2,600 fps. Calibers which are built around cartridge casings of higher volume than the wimpy .223 Remington enjoy much greater powder increases." receptive and helpful in my quest. I am sure they could see the market potential for profit for their companies as well.

Polygonal rifling, 5R, even numbers of lands, odd numbers, you name it. Nothing was 100% reliable in retaining the thin Sierra 90 grain jackets at a M. V of 2,650 fps. Had I been willing to back down my muzzle velocity by 100 to 150 fps. and just use the bullet for 600 yards, I believe some of the barrels might have worked. Doing so would have been contrary to my vision of what was good for the Reserve Team, which was one gun with one load capable of shooting winning scores at both 600 and 1,000 yards.

I still believe that goal is compatible with the needs of civilians as well. Most civilians have limited training time and can’t afford to splurge $3,000 on a custom AR-10 to shoot a couple of times a year any more than the Reserve Team could. While there were only three AR-15 90 grain barrel choices available originally, there must be about a dozen now American Gunsmithing Association

and all came into being due to Army Reserve Marksmanship Program involvement. Switching To Bergers Reluctantly, I abandoned the Sierras and approached Berger. Berger, much like most of the barrel companies, was very receptive and could see the potential market. They were already making a 90 grain VLD. I shared with them the problems everybody was having with the jackets of the Sierras and we agreed that a thick jacket would be the ticket. I didn’t have to wait long. The first 90 grain Berger B.

T. bullets even had the words "Thick Jacket" written on the boxes to let shooters know it was not going to blow up. I torture tested the new bullet to death and couldn’t get them to give. The Berger 90 grain B. T. was a little more aerodynamic than the Sierra and had a higher B. C. Berger recommended a slightly slower start velocity of 2,600 fps to keep the bullet supersonic and strong at 1,000 yards, about 50 fps slower than the suggested Sierra velocity.

Initially, I was a bit surprised when I found my primer retention to be more of an issue with the Berger while hitting only 2,600 fps than with the Sierra at 2,650. Most of us would probably expect the reverse of that to occur. I figured it could either be a bearing surface issue with the Berger or perhaps the thick jacket.

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So, the Berger bullet was 100% reliable with regard to jacket retention, accurate, but gave me chamber pressure problems at anything over 2,500 to 2,550 fps. depending on the specific barrel in question. By this time in history Bartlein Barrels, Inc. (bartleinbarrels.com) was making gain twist barrels, dubbing them "Transitional Rifling" and making them in either a 5R or a four groove. They can also supply .22 center-fire barrels with either the traditional 0.219" land or the tighter 0.218".

While taller 0.218" lands pressure up a little more, the accuracy offset is well worth the small sacrifice. I find the 0.218" land to be more accurate with all bullets I have tested. Back when I was still working with the Sierra 90 I had sat down with the plant manager from Krieger out at Perry one day for about an hour and brain stormed ideas with him on ways to improve jacket retention. He had recommended cutting the groove just a tiny bit deeper than normal, 0.2245" versus 0.2240".

That helped with jacket retention but still didn’t cure the problem. I noticed, however, that the slightly deeper groove helped cut pressure with no sacrifice to accuracy. I have carried that lesson with the Krieger barrel (they use the tall .218" land also) forward and order my Bartlein Transitional Rifled barrels with the same 0.2245" grooves. As stated previously, my rate of twist starts at 1:13" at the throat and doubles to 1:6.5" at the muzzle. Several factors conspire to create high chamber pressures.

The first is accelerating the bullet forward. Weight of the bullet, powder charge, burn characteristics and other ammunition variables come into play. Throat lengths of the chambers, groove depth and width, land height and width must also be considered. The second factor is rotating the bullet. Slower twist rates at the front of the chamber result in less resistance to bullet rotation and lower pressures.

Using my gain twist 1:13" x 1:6.5" configuration and the decreased pressure that results, I can dump about ½ grain more powder into my cartridges which brings my velocity to the target 2,600 fps. I am told that calibers which are built around cartridge casings of higher volume than the wimpy .223 Remington enjoy much greater powder increases, up to three full grains in some examples like the .338 Lapua with big cartridge casings.

Reloaders are cautioned to proceed GRADU- ALLY upward from published safe loads, however, as each gun will be different. Err on the side of safety! RKS Barrel up in Canada makes six groove gain twist blanks which mirror the specs for the Bartleins with similar reductions in chamber pressure while maintaining fine accuracy. Both companies will make blanks in either stainless or chrome molly. There is not much difference in price between the US source and the Canadian but the wait periods are different.

My usual wait on the Bartleins is six to twelve months while RKS delivers in one to three. We have established that gain twist reduces chamber pressures allowing more powder to be loaded resulting in increases in velocity. We have found that getting the bullet rotating sooner and more easily can result in less stress during obduration. Next time we will look at more applications and advantages of gain twist barrels. AG