The 1:8 barrel twist

[ta0117]

As stated above, even though ball propellants are double based, they have some of the lowest flame temperatures and are the easiest om throats.

The propellant used in the M855A1 is SMP-842, a variation of CFE223.

Is this SMP-842 a low temperature powder? Can’t find data on it.

[lysander]

Is this SMP-842 a low temperature powder? Can’t find data on it.

It is a ball propellant so it should be similar to other ball propellants.

[constructor]

Im not following here, are you saying 1/8 gets more FPS because of less rotations or is it some kind of special rifling combined with a slower twist that does it. Is this like a Triarc track barrel that has some kind of special rifling that boosts FPS or something? I believe the track series barrels are also 1/7 so Im guessing the mfg'er doesnt 1/8 would benefit the barrel.

Rifling design, less engraving force and less resistance. Most 1:7 is plain square enfield rifling with 40:60 or 50:50 l-G ratio. Some of the Triarc barrels were produced by us.
The 7 twist buttons are a military design so most just use them as they are. Military didn't design the 8 twist buttons so whoever uses them must design the button. If they have enough sense to use 8 twist then they usually design an improved rifling. 5R, Shilen's ratchet, like Noveske's poly but it isn't really a poly like HK or Glock it's really like a "R" type, Columbia's poly(female type poly) .

[markm]

Military didn't design the 8 twist buttons so whoever uses them must design the button. If they have enough sense to use 8 twist then they usually design an improved rifling.

Maybe why we've had such glorious luck with and love for the Rem 700 1/8 5R rifle we've shot the piss out of for the last 10 years.

[Disciple]

The 7 twist buttons are a military design so most just use them as they are.

Do you know if Ballistic Advantage Hanson barrels use the military design?

[constructor]

Do you know if Ballistic Advantage Hanson barrels use the military design?

No, Just borescope it to see if it has square lands

[DwayneZ]

Slower twist theoretically would give higher velocities for the same pressure profile as less energy is used to rotate the projectile. It was easily measured in old tank guns.

Indeed true, but by how much? Rainier Ultramatch 1:7.5 20" vs say a Wilson 1:8 20" or even some oddball 1:12 twist barrel. Same round, not sure a chrono is gonna pick up the diff.

I think the best config is achieving just enough rpm's to stabilize bullet, from there you have best balance between rpm's and FPS. In that math you can also look to using minimal twist to achieve needed rpm's to stabilize bullet.

For readers, powders transfer energy into the bullet, two types of kinetic energy, 1) velocity FPS, and 2) rotational or angular velocity (rpm's).
By way of energy conservation, the energy transfer to muzzle = FPS + rpm's.
Less rpm's means a tad more FPS. More rpm's means a tad less FPS.

[Disciple]

No, Just borescope it to see if it has square lands

I don't have one.

[constructor]

Indeed true, but by how much? Rainier Ultramatch 1:7.5 20" vs say a Wilson 1:8 20" or even some oddball 1:12 twist barrel. Same round, not sure a chrono is gonna pick up the diff.

I agree, all factory ammo is lower than max pressure so it's doubtful there will be much noticeable difference.
If you had a 12 twist 5R and worked up to a maximum load in it then stuck the same load in a 1:7 twist with 6 grooves and a 40-60 L-G ratio you would probably see a difference. 4-5000psi isn't going to blow the barrel but it may flatten or pierce the primer. Anyone looking for all out performance should look for a barrel with better specs, if taking carbine courses with factory ammo it's not going to matter.

ETA- One benefit - If you are using a SBR and you lose terminal performance due to loss of velocity, a better spec barrel may help gain some velocity and terminal performance. When we were doing all the testing in 2008 we found the 5R Melonite treated barrels were getting 60-90fps more out of the same length(16") barrel, our 12.5" barrels were as fast as some 16" barrels.

[constructor]

Maybe why we've had such glorious luck with and love for the Rem 700 1/8 5R rifle we've shot the piss out of for the last 10 years.

Boots knew what he was doing, funny thing is his design was based on a Russian design.