Polygonal rifling in AR barrels

[R1100R]

I have been reading lately about polygonal rifling in AR barrels. I am thinking of a new

upper. Does anyone here own one or have any experience with this type of barrel?

This will be a 5.56 set up.

Edited February 13, 2011 by R1100R

[William_Munny]

Ive shot a Noveski (sp) barrelled AR that my friend has. He said that the barrel, because of polygonal rifling, can generate faster velocities... i don't buy it unless someone chrono's it and proves it to me....

... BTW, the shots felt the same, no difference in recoil but it did shoot well I managed close to a 1 inch group at 200 yds with four rounds on factory ammo.

[PaiMei]

Polygonal rifling supposedly extends the life of the barrel. Its also suppose to marginally increase muzzle velocity. It also creates a slightly thicker barrel as the lands and grooves, are not as deep as traditional grooves. It is also...a little easier to clean. That being said I don't think its a must have. If your looking for an accurate, quality, hardy barrel...Id look at anything thats cold hammer forged. Currently, BCM and Daniel Defense are making all theirs in house here in the US. If your looking for just pure precision, Id look at the Stainless barrels. There are however...conflicting reports on their life expectancy. But...thats a debate for another day.

[R1100R]

The barrel I am looking at is a blackened stainless barrel by Black Hole Weponry. The complete upper is here.

COMPLETE AR Upper w/ Quad Rail, Match BBL, BCG & Charging Handle [Complete Upper w/Quad Rail & BCG] - $499.00 : New Frontier Armory - Firearm Sales, Transfers, and Consignment

It has a 3R polygonal riifling. These upper have had some good reviews on the forums out west. It is my understanding that most polygonal rifled barrels are hammer forged. I am going to call them for more info.

[Guest BungieCord]

If polygonal rifling reduces friction against the bullet, which I presume it would, it should lower muzzle velocity, not raise it. Coated bullets (moly, WS2 or hBN) work the same way. Chamber pressure only gets as high as it must to to drive the bullet out of the barrel. Lowering friction means less pressure is needed. Lower pressure = lower muzzle velocity.

[R1100R]

I have read that it increases it. All most every review I have read states the same, From Wiki a short version

A number of advantages are claimed by the supporters of polygonal rifling. These include:

• Not compromising the barrel's thickness in the area of each groove as with traditional rifling.
  
• Providing a better gas seal around the projectile as polygonal bores tend to have a slightly smaller bore area, which translates into more efficient use of the combustion gases trapped behind the bullet, slightly greater (consistency in) muzzle velocities and slightly increased accuracy.
  
• Less bullet deformation, resulting in reduced drag on the bullet when traveling through the barrel which helps to increase muzzle velocity.
  
• Reduced buildup of copper or lead within the barrel which results in easier maintenance characteristics.
  
• Prolonged barrel life.
  

The majority of polygonal rifled barrels are hammer forged. This is common practice in Europe gaining intrest here.

[Guest BungieCord]

The gas pressure only ever builds up as high as it needs to to blow the obstruction -- the bullet -- out of the barrel. If there's no obstruction -- meaning no bullet -- the pressure will remain relatively low. If there's a rigid obstruction -- like a plug welded in the barrel -- the pressure will build until something fails and lets it out. Anywhere in between, the pressure will build according to the stubbornness of the obstruction. Low friction, low pressure. High friction, high pressure.

The improved gas check might aid acceleration enough to overcome the loss from the reduced friction but reducing friction definitely lowers MV. If they're claiming the reduction in friction is what increases the MV, they're blowing smoke up your skirt.

[R1100R]

My concern is not over a slight increase in velocity. It has to due with the increased accuracy with polygonal rifling. Noveske use a modified type of Polygonal.

My query was whether someone on here owns one or has shot one. Not a lesson in physics.

Something new is not always bad. This appears new to the AR platform. A couple of well know manufacturer's are now making this type barrel.

Edited February 16, 2011 by R1100R

[1gewehr]

The gas pressure only ever builds up as high as it needs to to blow the obstruction -- the bullet -- out of the barrel. If there's no obstruction -- meaning no bullet -- the pressure will remain relatively low. If there's a rigid obstruction -- like a plug welded in the barrel -- the pressure will build until something fails and lets it out. Anywhere in between, the pressure will build according to the stubbornness of the obstruction. Low friction, low pressure. High friction, high pressure.

Hmmm, this sounds a little fishy. If that were true, why would faster burning powders yield higher pressure than a slower burning powder using the exact same barrel/bullet combination? It seems to me that since burning powder has a sharp pressure peak just as the bullet starts moving, that reducing the friction would allow that peak to spread further down the barrel, leading to higher velocities. This works exactly the same as why friction-reducing rings in your car's engine leads to higher peak HP and better fuel economy.

[Guest BungieCord]

Sorry, R1100R, didn't mean to hijack. My final post in this thread.

I know it sounds bass-ackwards. I didn't believe it myself until I started shooting coated bullets. You lose a little MV and a whole lotta chamber pressure because it doesn't take so much force to drive them down the barrel.

This is a comparison of pressure graphs from two loads in QuickLoad. Identical bullet and identical charge except the bullet on the right is moly-coated, which QL refers to as "friction-proofed":

Peak pressures of the two loads is as shown. It's a bit hard to tell in the scaled-down image but in the original, the pressure and velocity curves clearly are steeper for the naked bullet than for the coated bullet. The naked bullet has a predicted MV of 3196 fps. Coated, it's 3175 fps. I've shot coated bullets from .204 to .308 and 20 fps is a pretty typical MV loss.

It's the same reason that a heavier bullet produces more chamber pressure from a smaller charge. The bullet is more resistant to acceleration (more from increased mass than increased friction) so it takes more pressure to expel it from the barrel. With a given charge, anything you do to decrease the resistance of that bullet to acceleration (which means either lighten it or reduce its friction against the bore) will decrease the amount of pressure that will have to build up behind it to blow it out of the muzzle.

[LngRngShtr]

My Dad bought an H&K 91 back in the late 80's early 90's it has poly rifling scary easy to clean and still accurate some 20 years later...

I know it is a .308 and it is recoil operated but the rifling is the same so the characteristics should be similar hope this helps

I am also looking at building an upper for an AR also and was looking into poly barrels but that project has stalled due to economy..

John