New Colt .308s

[Benito]

I wouldn't do anything slower than 1:10 on a 16" 7.62.
Other then that, there seems to be some promise to the guns.

Please elaborate, as I don't know much about .308 twist rates and how they correspond with various bullet weights for stability, etc.
At what distance would any issues arise at?

[Failure2Stop]

Please elaborate, as I don't know much about .308 twist rates and how they correspond with various bullet weights for stability, etc.
At what distance would any issues arise at?

FBI testing has concluded that nothing slower than 1:10 adequately stabilizes the newer generation of duty ammo at 16" velocities. They are getting longer, and the shorter barrel is making them slower, necessitating in increased twist. Don't be surprised when 1:7 7.62 is available.

Another issue is use with a suppressor. While 175s tend to be ok from 1:11 down to 16", there is an increased risk of baffle strike, exacerbated in certain conditions, and even more so with more contemporary projectile design.

There is no shortage of good groups with ammo and twist rates that are sub-optimal, but those exceptions do not disprove the data that has been collected over the last 25 years.

[markm]

FBI testing has concluded that nothing slower than 1:10 adequately stabilizes the newer generation of duty ammo at 16" velocities. They are getting longer, and the shorter barrel is making them slower, necessitating in increased twist. Don't be surprised when 1:7 7.62 is available.

Wow. 1/7 seems a little over the top... At our altitude (about 1400+/-) I just start to see instability with a 1/12 twist barrel and 200 gr bullets loaded to subsonic velocities.

It just strikes me that 1/9 would more than work for any duty loads out of a 16" barrel.

[Failure2Stop]

Wow. 1/7 seems a little over the top... At our altitude (about 1400+/-) I just start to see instability with a 1/12 twist barrel and 200 gr bullets loaded to subsonic velocities.

It just strikes me that 1/9 would more than work for any duty loads out of a 16" barrel.

It's not just external ballistics with the new projectiles, but terminal as well.
I can't readily share the data on how much difference there is terminally at moderate to long range with different twist rates, unfortunately, however I can say that the bottom line is that 1:10 is as slow as is recommended.

[markm]

I'm going to have to soak that in for a bit. I suppose a faster twist could equal deeper penetration before stability is lost.

[mildot]

So is this the old 901 with the 9-3 o'clock rails removed? or have they made other weight reductions?

[PatrioticDisorder]

It's not just external ballistics with the new projectiles, but terminal as well.
I can't readily share the data on how much difference there is terminally at moderate to long range with different twist rates, unfortunately, however I can say that the bottom line is that 1:10 is as slow as is recommended.

Dumb question, but if one were worried about terminal performance wouldn't an overly stable projectile be a bad thing?

[darr3239]

The basic model seems to be a non-monolithic rail system, as mentioned. It looks like they added a completely different type of monolithic rail, in addition to the original, for the LE models. The newer monolithic rail is stated to be a pound lighter.

http://www.colt.com/Catalog/Rifles/MARC901Series.aspx

[Failure2Stop]

Dumb question, but if one were worried about terminal performance wouldn't an overly stable projectile be a bad thing?

Apparently not, hah.
Again, not my testing in that department, but I'm making the presumption that the open or ballistic tip of these projectiles don't really care about stability, and that as soon as they begin to deform the gyroscopic force gets them open nicely.

On the stability side, on the external ballistics issue, the faster twist is good for the trans-sonic range.

I didn't mention this, and I really should have, but this testing was not limited to 16" barrels, and some issues are more pronounced in shorter barrels.

[domestique]

Much nicer looking then the original hand guard.

The twist boggles me though. At least do a 1/11".

I won't be trading in my LMT rifles anytime soon, but would love to have a spare 308 in the safe. Using the lower receiver as a dual purpose 5.56 18" SPR would be appealing as well.