I recently did some research on the matter for a member who wanted to have a polygonal gain-twist rifled barrel in the ABS carbon-fiber barrel.

I called Pac-Nor, and they have sold their gain-twist tooling.

The guy who bought it isn't doing anything with it, and we couldn't get him to let us use it.

Pac-Nor's statements on the matter were that maybe a 2% gain in velocity with certain loads might be attainable in an AR with the gain-twist.

Not worth the trouble.

Polygon rifling is another matter, and may have some benefits due to tight gas seal around the bullet. However, for maximum accuracy work, the 5R rifling and other match-grade cut rifling still seems to dominate the benchrest competitions.

Regarding longevity, I haven't personally conducted any tests to see how many rounds can be shot from a polygon-rifled barrel and still remain accurate. However, I will say that the accuracy deterioration generally comes from throat erosion, and it would seem that the rifling type would not have much effect on that activity. Maybe the harder steel would. But hard steel and rifling types are two different subjects.

DevL,

We are currently making a 10.5" SBR barrel in ABS Carbon Fiber process, using the SS Pac-Nor polygon-rifled blank in 1/7 twist, with a suppressor mount.

This is for an ARFcom member.

It is the same kind of barrel blank that Noveske uses in his barrels.

We can use any quality barrel blank with our ABS Carbon Fiber process, however price can vary depending on how much the blank costs, and the turnaround time is increased because it normally takes about 4 weeks to get a barrel blank from Pac-Nor or other barrel makers.

This polygon rifled blank option has always been available from us. We've made a few of them, but it hasn't become highly popular yet.

Anyone wanting the Pac-Nor polygon barrel done in carbon fiber can have it.

In fact, it may not be well known because I haven't advertised this, but ABS is quite happy to make regular(non-carbon fiber process) match grade barrels to order also.  ABS is a custom barrel maker, and they can make all kinds of barrels, using whichever type of barrel blank is desired.

Jmart,

To be honest, I have no chronograph data with these polygon barrels with any type of load.

Pad-Nor makes a very nice barrel, but I wouldn't expect anything too far out of the ordinary as far as velocity is concerned from a polygon barrel.

My gut feeling is that if you got an extra 100fps out of it, you'd be doing well. I don't think it is going to solve the short barrel/low velocity problem.

I think that there are some people doing some wishful thinking out there, who are hoping that it will.

The real truth is that a short barrel will reduce muzzle velocity with almost any rifle cartridge.
The best you could hope for are minor improvements.
The straight answer is to get a longer barrel, if you want more velocity.

I know that there is always a hope for some "magic potion" that will solve everyone's problems, but that isn't really going to happen with this short barrel/low velocity issue.

I think the answer to the issue  lies in bullet design, because that is far more hopeful than getting long barrel velocities out of 10.5" barrels.

If the right bullet design came along which could have the correct terminal effects at the low velocities that these short guns produce, then we'd have less of a problem. But, we'd still have the larger amount of bullet drop , and lower energy figures at relatively shorter distances from these short-barrelled guns.

So, while I like the Pad-Nor barrels and the polygon rifling just fine, and you may get a small velocity bump, I'm not looking to those things to solve the basic problems that we are facing in the SBR world.

Regarding conventional(non-polygon) rifled barrels, you have to remember that there are "fast barrels" and "slow barrels" in the normal barrel world. I've seen what appeared to be identical barrels shoot 100fps different velocities with the same bullet/load. It is really only accurate to say what a particular barrel is doing, because there is quite a bit of variance in what any given barrel will do, compared with another barrel, in terms of velocity. A good gas seal, good chamber fit, and a good fit of the bore to the bullet, and a well-lapped bore will definitely help to improve the velocity results out of any barrel.

Austin,
I think you could help us all out when you get your barrel, by chronographing some 77gr stuff, and letting the membership know what velocities you are getting out of the 10.5 polygon Pac-Nor.

We're working on getting it to you ASAP.

My "seat of the pants" calculations tell me that if the 77gr load will do 2700 out of a 16", it should do about 2500-2550 out of a 10.5. If you add the 100fps from the polygon barrel, that puts it at about 2600 - 2650fps.

That's my ballpark estimate for that load in a polygon 10.5'.
And, when you consider that this load has only a small velocity margin over fragmentation velocity to start with, a 100fps boost would be significant in extending the fragmentation range, especially with a good BC bullet like the 77gr.

So, when you consider that it needs all the help it can get, that extra boost would be valuable to extend frag range from the SBR.

I'm not knocking the polygon barrel.
I'm just saying that a 10.5" is cutting it pretty slim in the velocity department.

Jmart,
You may be right.

I'm only making crude estimates at best, with my figures above.
No claims being made.

I don't use anything shorter than 14.5" myself, and I find myself leaving the 14.5" in the case more and more now, in favor of using my 16".

Again, there are always "fast barrels" and "slow barrels" no matter what you do. If you get a slow barrel, then even if the rifling did make a difference, you might never know it.  If you get a fast barrel, then you might think the rifling did alot more than it really was doing.

I'd say again that it only matters what any one particular barrel will do, and that's the one you are using at the time.

I have wanted to work with gain twist barrels for some time, I think there could be some pretty amazing results with regard to velocity, accuracy and pressure.  

Polygonal rifle does improve velocity and reduces pressure slightly.  

What about squeeze bore technology?  

Very short barrels and superlative internal ballistics, two benefits of squeeze bore technology.