M4 velocity question

[cmdc]

I have a number of ARs and other 5.56 caliber rifles, and most of them are 1:8 or 1:9 twist. I also have two 16" carbines with 1:7 twist(Colt 6920 and KAC SR 15). I am wondering how much loss of effective range there is in these rifles with heavier bullets compared to a 20" barrel. Thanks

 

[holdencm9]

http://www.ballisticsbytheinch.com/223rifle.html

They don't have 20" barrels tested yet, but going from 16 to 18 only increases the velocity of the 55gr pill by a couple percent. I have heard some people state that you lose 100 fps per inch, but I think that is incorrect, it is more like 25-50 per inch, at least at the higher end of the spectrum (velocity does not increase linearly...most of the acceleration is at the beginning, so going from 20 to 16 loses a lot less than going from 16 to 12, etc.). RC had referenced a study here that seems to say the same thing, for 55gr bullets. http://www.thehighroad.org/archive/index.php/t-352353.html

As for the heavier bullets, I don't think they would experience that much more significant loss of velocity. The 1:9 may not stabilize the bullet at all, or if it does, it may become unstable at longer ranges. I've heard some people's 1:9's stabilize the bullet just fine out to 100 yards for instance, but beyond that they may start to get keyholing or just poor accuracy. The 1:7's should be stable for the entire trip. In essence, any velocity loss by going from a 20" 1:9 twist, will be balanced out and then some by the 1:7 twist in the 16" AR. So the bullet might lose 5-10% of velocity, but it will be spinning over 20% faster, per fps.

 

[taliv]

M193 should give you around 3200-3250 fps from the muzzle of a 20" weapon and around 3150 from a 16" weapon.

M855 should give you around 3050-3100 from a 20" weapon and around 2950-3000 from a 16" weapon.

from the ammo-oracle archive

http://www.razoreye.net/mirror/ammo-oracle/AR15_com_Ammo_Oracle_Mirror.htm#ballmil

 

[cmdc]

Thanks for the information, gentlemen.

Interesting read taliv, thanks for the link.