Another Physics Question

[Dionysusigma]

Does the actual shape of the rifling affect performance (accuracy/ relative speed) in a firearm at all?

Polygonal vs. traditional lands and grooves vs. "rounded" inverse-grooves (whatever they're called)...

I can see how a somewhat soft all-lead bullet going through a polygonal rifled barrel would have a better gas seal than a good amount of anything else out there; thus, a potentially higher speed for the same charge... if the friction from the added contact doesn't slow it down too much.

Any thoughts?

 

[Jim Watson]

I think you can make a case that the rounded off rifling commonly termed "polygonal" and used by such makers as Glock and H&K will average slightly higher velocity due to less friction and lower engraving forces than land-and-groove rifling.

I think you can also make a case that land-and-groove rifling averages better accuracy. I don't know any target shooters who use rounded rifling. And surely somebody would if he thought it would gain him a point or so on the target. H&K made some polygonal sniper rifles, so it isn't bad, it just isn't great.

I think the main thing about polygonal is that the gradually contoured cross-section likely gives the maker a stronger, longer lasting mandrel for his Krupp hammer forge barrel machine than one with a lot of square corners. Follow the money.

 

[MachIVshooter]

Since acceleration is basically the medium between driving force and resistance, lowering the resistance will increase the acceleration. Pollygonal rifling does offer lower resistance, hence slightly higher bullet speed. AS an example, my 4.7" barreled poly-rifled Baby Eagle averaged 1207 FPS with 115 grain bullets. The same load fired from my Beretta 92 FS with a 5" L&G barrel averaged 1165 FPS. The increase is relatively marginal, but present.

AS for accuracy, I would think the type of rifling is far less critical than all the other factors that make a firearm accurate. I doubt one type has any advantage over the other, all else being equal and both barrels being good quality.

The biggest advantage to polygonal rifling is ease of cleaning.

 

[Molon Labe]

Does the actual shape of the rifling affect performance (accuracy/ relative speed) in a firearm at all?

Absolutely.

There's a lot more to rifling than meets the eye. There's twist rate, number of grooves (2 through 8), groove depth, type (rounded, deep, parabolic, Mannlicher, Metford, Lancaster oval, etc.), gain twist, decreasing depth, etc.

Robert Rinker devotes an entire chapter (Chapter 12) to the subject of rifling in his book Understanding Firearm Ballistics.

 

[esmith]

I hear you can have a bullet traveling through a barrel that its twist rate was designed for x amount of grains. Is it true that if you use an extremely lighter or heavier bullet that all accuracy will be lost?

 

[thexrayboy]

It is quite true that a barrel length and twist rate that works well with a specific bullet will be abysmally inaccurate with another. The original
M16 rifles had a 1in12 twist and worked well with the 55gr round. When they stepped up the weight of the round they started seeing rounds keyhole the targets out to about 100 yards and miss altogether at longer ranges. The newer versions of the M16 had faster twists such as 1 in 9 and 1 in 7. The 1 in 9 is a good compromise and can handle most bullet weights. The 1 in 7 will spin the round at an amazing speed and smaller light weight varmint rounds like the 40gr can deform or even fly apart due to the centrifugal forces.