More Testing On Bullet Effectiveness And Barrel Twist Rate
BY Herschel Smith
3 years, 4 months ago
I say more in the post title because there’s already been a lot of that testing. Watch Andrew’s testing and listen to his points all the way to the end, and then I have some remarks.
We’ve discussed this many times before, and linked various articles on this: Small Caliber Lethality: 5.56mm Performance in Close Quarters Battle (note that links to this paper in previous posts are broken but this one works), 5.56X45mm Versus 7.62X39mm, AR-15 Ammunition and Barrel Twist Rate, and a CFD analysis entitled Prediction of Projectile Performance, Stability, and Free-Flight Motion Using Computational Fluid Dynamics.
I had never even once believed that the reason for a change of barrel twist had to do with yaw inside tissue. The real reason is found elsewhere.
Accuracy cannot be assessed without addressing the rifle barrels’ twist-rates. In the early 1980s the M855’s 62-grain bullet was developed for the M249 Squad Automatic Weapon (SAW). For purposes of interoperability, the same load was adopted as the M16A2 rifle’s standard ball as well. A February 1986 U.S. Army study noted that the M855’s bullet required a “1:9 twist [which] would be more appropriate for the M16A2 rifle, improving accuracy and reliability.” Multiple studies confirmed the 1:9-inch twist requirement.
But then a problem arose. The U.S. military’s standard M856 5.56 mm tracer round was longer, heavier (63.7 grains) and slower than the M855 ball, and simply would not stabilize with a 1:9-inch twist barrel. Thus, despite it doubling M855 group sizes, the M16A2 (and later, the M4) specified a 1:7-inch rate-of-twist barrel to stabilize the tracer round. It remains so to this day. Therefore, M855A1 was test-fired with both 1:7- and 1:9-inch twist barrels, and it was verified that this new cartridge is consistently more accurate in the latter barrels-as was its predecessor.
Note that in these articles I’ve also cited contacts in the industry who claim that this concern is a bit overblown, and that a barrel twist of 1:9 is perfectly sufficient to stabilize bullets up to and including 62 and 65 grains, and even 77 and 80 grains. Some of this has to do with barrel manufacturing procedures and quality.
I think Andrew just confirms what we already knew.
On June 17, 2021 at 2:09 pm, A Nonna Moose said:
Interesting, but it’s also important to remember what happens when you run 55gr ammo in a 1:7 barrel. It spins that light bullet so fast that it comes apart. The military specifies M193 ammo in an M16A2 for use in emergencies only.
On June 17, 2021 at 11:06 pm, X said:
“I’ve also cited contacts in the industry who claim that this concern is a bit overblown, and that a barrel twist of 1:9 is perfectly sufficient to stabilize bullets up to and including 62 and 65 grains, and even 77 and 80 grains. Some of this has to do with barrel manufacturing procedures and quality.”
It’s not overblown. It has to do with the ballistic coefficient and the length of the bullet. I can stabilize a 70-grain Speer round nose flat base in a 1 in 12 twist for stupendous accuracy at 100 yards… but not a 62-grain M855 spitzer boattail.
The military required a bullet that could shoot out to 500 meters, and shoot point-blank onto an human silhouette out to 300 with their 25-meter zeroing targets. The 70 grain round nose flat base simply won’t do that.
A 1 in 9 barrel will shoot 69 SMKs or 75 Hornady HPBTs with stunning accuracy, but not VLDs like the 75 A-Max or the 80 SMK because they’re too long.