5.56 - How much velocity do you lose per inch of barrel?

[DannyinJapan]

I read somewhere that the shorter the barrel, the slower the round, which is a basic truth, but I wondered if someone could tell me exactly how much velocity I can expect for each barrel length? (a reasonable estimation, anyway)

14.5"
16"
17"
18"
19"
20"

And what would you say is the shortest length a combat rifle ought to have that would still preserve the high velocity that the 5.56 round requires to make it effective at 50+ yards?

 

[rcmodel]

http://counterstrikefox.freeservers.com/mv.htm

Personally, I think the 1/7 twist 14.5" M4 should have been a 1/12 twist 16" if killing power was the only measurement or requirement.

rc

 

[Matrix187]

^ Would that make the bullet unstable in flight possibly making it do more damage at close range?

 

[rcmodel]

No, it would be perfectly stable in flight.

But it sure wouldn't be when it hit someone.

I think part of the problem with the current M16/M4's poor reputation is the 1/7 rifling twist making the bullets too stable.

rc

 

[MTMilitiaman]

Bullet acceleration is not linear. The majority of the bullet's velocity is achieved within the first several inches of barrel length. Thererfore, you'll lose less velocity going from a 20 inch barrel to an 18 inch barrel than you will going from an 18 inch barrel to a 16 inch barrel, from a 16 inch barrel to a 14 inch barrel, from a 14 inch barrel to a 12 inch barrel, and so forth.

The general rule of thumb is 30 to 50 fps per inch of barrel, but this is a "general" rule.

Going to a 1:12 twist 16 inch barrel would have absolutely no positive effect on either "killing power" or accuracy. This twist rate is unlikely to stabilize anything above 55 gr, esp in colder, more dense atmospheric conditions, but this is not a good thing. Even with the standard 1:7 twist rifling, it takes the 62 gr M855 ball round almost 30 yards to fully stabilize. This effect has been filmed using high-speed photography and radar at Aberdeen Proving Grounds, and can not be contested. Therefore, going to a slower twist rate would have minimal advantages in terminal effect within the range it retains acceptable accuracy as within this range, the faster twist still hasn't achieved full stability either. It is, however, a common fallacy that the military's decision to go to the faster twist is responsible for some mythical loss of terminal effect within the cartridge.

http://ammo.ar15.com/ammo/project/perf_twists.html
http://ammo.ar15.com/ammo/project/perf_112.html
http://ammo.ar15.com/ammo/project/perf_m193acc.html