is there any difference to shooting polygonal barrels? Will it make the bullet more accurate?
it’s for an 18in spr barrel.
any advantage? disadvantage?
Better gas seal & slight increase in velocity.
Correct.
Additionally, polygonal rifling results in less bullet deformation (which can aid in accuracy, however slightly), enhanced barrel longevity, and greater ease of cleaning.
AC
so far an 18in Barrel Build. (larue stealth upper) This is good right? I mean I can get more velocity out of my load.
any disadvantages? cleaning? how come others don’t make polygonal if it’s good?
HK uses polygonal rifled barrels so they must be better and they will even shoot bullets that are placed backwards into the mag! 
I do like the polygonal bbl in my USP45 because its so easy to clean.
yes. I’m aware of polygonal in pistols like HK and Glock.
I was just wondering how it affects or matters in rifles.
More than one barrel maker user polygonal rifling. I suspect it costs more to make which is why it is not the most prevalent sort of barrel. Just a WAG
oh?
If it’s so good how come SPR builders and accuracy nuts don’t go for polygonal?
I believe Noveske Stainless bbls are polygonal.
The fact is that many premium barrel makers DO use polygonal rifling – or a derivative of it, such as the Ratchet Rifling commonly used by Shilen.
Personally, my interests run toward Noveske or Rainier Arms, as the latter uses Shilen blanks.
AC
Because it’s better in terms of durability and ease of cleaning than it is in terms of raw accuracy.
If it really were that advantageous from an accuracy standpoint, the benchrest guys would be all over them.
Before this starts down the path of the anecdotal, it is worth observing that we’ve entertained this same discussion previously, and there are a number of variations on polygonal rifling. Going back to my earlier remarks, neither Noveske nor Shilen/Rainier use a pure polygonal profile, and I’m sure that many of the other SPR barrel makers don’t either.
The benchrest crowd may be concerned with extreme accuracy, but they are using very different rigs, very different loadings and have very different constraints. To suggest that there is a direct translation from one discipline to another is, in my view, an oversimplification. We do know what works in SPR applications in the AR world, and most of the top tier operations have chosen to use semi-proprietary rifling techniques that may or may not fit neatly into the categories we usually correlate to production arms.
Rather than expend a lot of energy debating polygonal verus traditional rifling, it would be more instructive to consider the results seen in the field from the best-in-class SPR rigs, such as those mentioned above. There are simply too many different variables in play to rule definitively for or against a particular approach, as two very different formulations can often deliver much the same practical results.
AC
I agree. To that end, though, I’m convinced that the accuracy exhibited by such barrels (like the Shilen, with its “ratchet” rifling, and stainless Noveske barrels, with their semi-polygonal rifling) is a product of the generally high standards of their manufacture, not a trick of the rifling. Polygonal rifling is not without its virtues, I just don’t happen to believe that inherently better accuracy is among them. If you look at the results delivered by some of the higher quality examples of the various rifling types, you’ll see pretty similar results across the board.
Accuracy aside, I think we can pretty clearly say that polygonal or semi-polygonal rifling is a good choice for an SPR, as it’s far easier to clean after high-volume shooting.
Agreed on all counts.
AC
ok… semi polygonal or polygonal higher velocity. which is good. It is of better quality.
but why do you say it is easier to clean? do we clean it the same way? nothing different?
To the best of my understanding, it’s because the angles of the lands and grooves tend to be more obtuse than the ~90° ones found in regular rifling. There are fewer places for the fouling to “hide.” If you look at a conventional bore, it can be damned hard to get the last bits of copper residue out of the sharp angle where the land and groove meet, since the bristles of the brush don’t get into that area very well.
There may be differences between the rigs, but when you are talking about accuracy, the similarities between the benchrest barrel and the AR15 barrel are tremendous. Both rigs push a bullet down a tube. A good barrel does as little damage to the bullet as possible, and vibrates consistently. Is that an “oversimplification” irrelevant to SPR barrels?
But as Crazymoose observes, the shape of rifling is only one factor in accuracy. In my opinion, it is a small factor. Chamber precision, crown consistency, smoothness and consistency of bore and rifling, residual stress, the number of lands/grooves, even barrel profile are more important than the shape of the rifling, IMO.
I own two 16" Noveske barrels with polygonal rifling. I have shot them from the bench using top-quality brass, powder, bullets, rests, and bags and benchrest techniques. A lot. They shoot well and clean easily. They are excellent M4 barrels. But my hand-lapped 20" Ar15 Lilja barrel (with only three grooves), which is made to true benchrest standards, is much more accurate, and more suitable for an SPR.
For expert insight into rifle barrels see: http://riflebarrels.com/articles/barrel_making/rifle_barrel_accurate.htm Why would any of the points mentioned be irrelevant to accuracy in an AR15? Are bullets shot from SPR’s unaffected by the quality/precision of the barrel?
ok… Polygonal barrel… is a product of quality.
but with regards to cleaning…
remember HK and glock pistols.? they don’t allow shooting lead even if you clean every 50 rounds. why ? because when you clean it… your brush can’t get to the lead in the corners…
so why is it that it’s easier to clean a polygonal rifle barrel compared to a polygonal pistol barrel?
I would agree with KYPD that the fundamentals don’t differ much from one application to another (i.e. benchrest to tactical SPR); however, we’re talking about highly-optimized rigs for very different purposes – or more correctly, rifles that may loosely share the same basic purpose, but which do so under wildly different conditions.
A highly-tuned benchrest rig with a 2 ounce trigger and a much-babied match barrel in a glue-in stock shooting special hand-loaded fodder is not well-suited to tactical applications. It achieves extreme accuracy to be sure, but it does so at the expense of things like durabillity, interoperability, flexibility and sustainability. There is no doubt that we have built sniper rifles to a similar formula for many generations, but I would argue that an SPR is not a sniper rifle in the traditional sense. What we’re really talking about here are accuracy-optimized ARs, as the shared benchrest-sniper DNA is more readily going to be found in rifles like the M24, TRG-42 -or even the PSG-1 – than in an SPR-barreled AR.
Some may see this differently, of course, and I take no issue with that. I would, however, advance the view that an AR-pattern SPR is first and foremost an AR, and as such, it must be durable, able to digest a variety of loads, and offer repeatable results under less-than-ideal conditions. Accuracy, while critical, is merely one component in this equation, as an accurate AR will serve little purpose in the field if it cannot withstand a certain amount of abuse, has no parts commonality with existing ARs, or cannot digest a variety of factory match loads.
To address your other question, I believe that the reason lead bullets are a bad idea in certain autoloaders has more to do with residue and its effect on the operating system (the P7 comes immediately to mind) than it does with the kind of rifling used in the barrel. Conventional wisdom has suggested that polygonal rifling contributes to the fouling problem, but would question that. I suspect that the softness of the lead used (especially in reloads, which no manufacturer is ever going to recommend) likely has more to do with this than anything, but that is really another discussion best left to others.
AC
so what’s a decent group size for an AR15 spr? 1MOA? or .75 MOA? or .5MOA???