.223 75 Grain Rifling

[4 Liberty]

If 1:9 rifling handles 55 gr. in an 18 in. barrel (Ruger) and 1:8 rifling handles 62 gr. in a 16 in. barrel (Smith M&P 15); each giving 2 FULL TURNS, then WILL A 1:9 RIFLING, 24 INCH BARREL, HANDLE A 75 GR. BULLET BEING THAT THE BULLET RECEIVES 2.7 FULL TURNS?

I know that popular logic says that 75 gr. .223s should be shot out of a 1:7 barrel BEING THAT THE BARREL IS 16 INCHES. However, I believe a 24 inch barrel on a sniper rifle with 1:9 rifling will adequately stabilize the 75 gr. bullet, giving it 2.7 turns before exiting. Am I right or wrong?

(Remember, the original Viet Nam AR-15 shot 55 gr. bullets all in an 8-inch circle at 300 yards and the rifle had a 20 in. barrel with 1:14 rifling. The barely stabilized bullets tumbled like the bigger, newer .245 mm AK-74 bullets, with both bullets tearing big holes.)

 

[davejones]

"I know that popular logic says that 75 gr. .223s should be shot out of a 1:7 barrel BEING THAT THE BARREL IS 16 INCHES."

As far as I know the rifling twist needed is based upon the length of the bullet, not length of the barrel. The additional length will give more velocity which might be needed if you're on the edge of stabilizing a particular length of bullet.

I don't think that the amount of full turns that a bullet makes while it's in the bbl makes any difference. I've shot more than one .223 w/a 10" bbl that were 1 in 14 twists that have agged around 1" at 100 yds. Another case in point is Smith revolvers in .357 cal, the twist on those was (is?) 1 in 18 3/4". They shoot fine w/only a 4 " bbl.

 

[4 Liberty]

Reply

"I know that popular logic says that 75 gr. .223s should be shot out of a 1:7 barrel BEING THAT THE BARREL IS 16 INCHES."

As far as I know the rifling twist needed is based upon the length of the bullet, not length of the barrel. The additional length will give more velocity which might be needed if you're on the edge of stabilizing a particular length of bullet.

I don't think that the amount of full turns that a bullet makes while it's in the bbl makes any difference. I've shot more than one .223 w/a 10" bbl that were 1 in 14 twists that have agged around 1" at 100 yds. Another case in point is Smith revolvers in .357 cal, the twist on those was (is?) 1 in 18 3/4". They shoot fine w/only a 4 " bbl.

Thanks for your upgraded info. According to this formula I get a twist of 9. You may recognize the formula. I used one inch for the length of the 77 grain .224 bullet.

Twist=CxD(squared)/L x square root of SG/10.9

where:
• C = 150 (use 180 for muzzle velocities higher than 2,800 f/s)
• D = bullet's diameter in inches
• L = bullet's length in inches
• SG = bullet's specific gravity (10.9 for lead-core bullets, which cancels out the second half of the
equation)

Another quote is from Barnes:

22470 .314 TSX BT70 70gr
*Recommended twist rate is 1:8 or faster for #22470

I think it is a mathematical but not perfect science. You showed that it's how the bullet shoots that counts.

 

[mike in co]

it is about bullet length.

not bbl length.

but not all 75 gr bullets are the same..take 75gr hp vs a 75 gr amax....

some 75's work in a 1/9 but not all....
if i was gonna shoot 75's i would go with a match bbl in 1/8 or 1/7...really depends on what else you plan on shooting.....

anyone want to show me a factory original m16 with a factory 1/14 twist bbl ???

( i think someone may have confused these twist with the factory colt in 222 rem..just a guess)

i have "heard" some 1/12's then jumping to 1/7 for the 64 gr tracer..sorta over kill. but it was guite long for its weight.

mike in co

 

[MilGunsmith]

The original XM16 was a 1-14" twist. It worked ok with the M193 55gr ammo, but had problems with stability during cold weather testing and lower velocitys. That is why they were built with a 1-12" twist. When the M855 62gr ammo came out, copying the Belgium SS109 ammo requirements, we put a 1-7" twist in it to match the Belgiums. This is actually too fast a twist for both the M855 and the slightly longer M856 tracer. The Belgiums were concerned with the tumbulling that was happening with the M193 and 1-12" when it hit tissue, and violating the Hauge convention, that they increased the twist to 1-7" too avoid this. The Canadians used a 1-9" in most of their rifles with M855 in cold weather without a problem.

Rotational speed is where the stability comes from. We experimented with 77gr SMK to see how fast we could shoot them. We started with M262 and a 20" 1-7" test barrel at 2800fps, we got good groups and stability. We rechambered to .22-250AI and were able to reach 4000+fps, but had high chamber pressures. The bullets were over stabilised and started to come apart, similar to the early .220 Swift problems. We switched to a 1-9" barrel with same loads and got good groups and no bullet problems. In reducing the loads back to 2800fps, in the 1-9" barrel, the groups opened up and bullets showed signs of tipping.

 

[mike in co]

so xm16 is pre production and m16's had 1/12...correct ???
i "heard" that the squad automatic weapon came out and tracers added to the line, m16's had issues with tracers in the 12 twist..and that was when twists changed...yes ??
thanks
mike

 

[davejones]

"anyone want to show me a factory original m16 with a factory 1/14 twist bbl ???"

mike in co, want to show me in my post where I said anything at all about an M-16?

 

[MilGunsmith]

The twist didnt change to 1-7" untill the M16A2 and the M249 SAW. The M16 and M16A1 both had 1-12" and both fired trace in addition to 55gr ball.

 

[mike in co]

dont get so paranoid....the original question has it in his post...try reading....
mike in co

"anyone want to show me a factory original m16 with a factory 1/14 twist bbl ???"

mike in co, want to show me in my post where I said anything at all about an M-16?

 

[Larry Elliott]

As others have pointed out barrel length doesn't have much to do with bullet stabilization as long as the barrel is long enough to give enough velocity. More barrel length is the only thing that will get the bullet moving fast enough if the twist is marginal. That's up to the point where lengthening the barrel doesn't provide much velocity increase.

I've got a .223 with 1 in 9 twist that will stabilize 75 gr Hornady A-Max bullets that are long for their weight, but other barrels with the same twist might not stabilize them. If the temperature goes down enough and drops velocity enough that barrel might not stabilize those bullets.

If in doubt try the bullets in the barrel, they'll let you know pretty quickly whether they'll work or not.