The evolution of the 1;7 twist rate on the M4

[Tripplet918]

The evolution of twist rates

Early M16s used a 1:14 twist rate as they fired light and short bullets. This rate was very limited in what it could do; not even the 55-grain rounds used were spinning fast enough. The military then moved up to 1:12 twist that put a better spin on the bullet. As heavier rounds came out, the twist rate became tighter and tighter. The 1:9, the military found, was good still slow enough for the 55-grain rounds, but it was fast enough for 75-grain bullets as well. As barrels became shorter, however, it was noted that even tighter twist rates were needed. This was because a bullet needs at least two rotations in the barrel to become stable. A 16-inch barrel with a 1:9 twist cannot deliver this, so the twist rate was tightened to 1:8. As barrels became even shorter and bullets became heavier, twist rates were again increased. During testing, however, it was noted that bullets that twisted too fast could tear themselves apart if the copper jacket had any imperfections. For this reason, the 10.5-inch barrels on AR15s still use a 1:7, sacrificing a full two rotations in order to not over spin the round and cause it to break apart.

https://blog.uspatriottactical.com/rifle-twist-101-a-basic-understanding/

[sixtringr]

Thank you,

I learned something useful today.

[CGZ]

IIRC The 1:7 twist rate came with the M16A2 and the adoption of the 62gr M855 "green tip" and the accompanying M856 tracer round. Something to do with the longer bullet length not stabilizing in the slower twist rates.

[Quiet]

IIRC The 1:7 twist rate came with the M16A2 and the adoption of the 62gr M855 "green tip" and the accompanying M856 tracer round. Something to do with the longer bullet length not stabilizing in the slower twist rates.

The 1980 NATO adoption of the Belgium SS109 cartridge, which the US Mil designated as the M855 cartridge.
^STANAG 4172.

During the 1970s, FNH determined that the Belgium SS109 cartridge required a 1 in 7" twist for stabilization out of a 13.7" to 20.0" barrel length.

[sigstroker]

M855 was similar, but not the same, as SS109. The cog was slightly different. supposedly it made it more lethal, but in my 1 in 12 Daewoo, the side effect was that it shot just fine at 100 yards.

[Quiet]

M855 was similar, but not the same, as SS109. The cog was slightly different. supposedly it made it more lethal, but in my 1 in 12 Daewoo, the side effect was that it shot just fine at 100 yards.

The South Koreans (Daewoo Precision Industries/SNT Motiv) went with a 1 in 7.3" twist on their 12" to 18.3" barrel length K-2 rifle series for use with their SS109/M855 ammo.
They used the 1 in 12 twist in their K-1 series of "submachine guns" and for all their rifles made for the civilian market.

[FLIGHT762]

1/7" twist was needed to stabilize the M-856 Tracer ammo. The original M-196 Tracer would trace up to 500 yards and would still work with the 1-12" twist in the M-16A1.

With the adoption of the new M-16A2, the Military wanted a tracer that traced out to 875 yards to go along with the M-855 ammo. In order to trace to 875 yards, the M-856 Tracer was a much longer projectile to hold the tracing compound.

The M-16A2 needed the 1/7" twist to stabilize the M-856 Tracer.

[bohoki]

"two rotations in the barrel to be stable" i call bs on that the rate is the "rate" if its 7 inches at 1-7 or 28" at 1 in 14

i think 1 in seven is close to the centrifugal destruction level of some projectiles

[sigstroker]

"two rotations in the barrel to be stable" i call bs on that the rate is the "rate" if its 7 inches at 1-7 or 28" at 1 in 14

i think 1 in seven is close to the centrifugal destruction level of some projectiles

The SIG Rattler in .300 Blackout is 1 in 5". The new Q 8.6 Blackout is 1 in 3".

Neither goes 3000 fps though. But Brittingham says the rotational energy of the 8.6 adds destructive force to the bullets. He's killed a bunch of big African critters with it.

[DrewN]

All I know is that regular 55's out of a 1/12 20" barrel work perfectly well for a variety of uses and have for 60 whatever years.

[sigstroker]

1/7" twist was needed to stabilize the M-856 Tracer ammo. The original M-196 Tracer would trace up to 500 yards and would still work with the 1-12" twist in the M-16A1.

With the adoption of the new M-16A2, the Military wanted a tracer that traced out to 875 yards to go along with the M-855 ammo. In order to trace to 875 yards, the M-856 Tracer was a much longer projectile to hold the tracing compound.

The M-16A2 needed the 1/7" twist to stabilize the M-856 Tracer.

The M856 doesn't really light up until it's nearly 100 yards downrange so that it lasts longer. That's in the daylight. At night it looks like it lights right out of the barrel. What was I doing shooting tracers at night? I had an inquiring mind that wanted to know. So I tested them out when it was not quite day and not quite night.

[sigstroker]

The South Koreans (Daewoo Precision Industries/SNT Motiv) went with a 1 in 7.3" twist on their 12" to 18.3" barrel length K-2 rifle series for use with their SS109/M855 ammo.
They used the 1 in 12 twist in their K-1 series of "submachine guns" and for all their rifles made for the civilian market.

That's what the online sources say about the military guns. That doesn't mean the retail guns were the same. I could swear my manual said my AR-100 that I bought brand new in 1989 said it was 1 in 12.

[Philthy]

That's what the online sources say about the military guns. That doesn't mean the retail guns were the same. I could swear my manual said my AR-100 that I bought brand new in 1989 said it was 1 in 12.

My recollection is the early imports were 1/12 and at some point changed to 1/7. The post ban DR?s are all 1/12.

[LAKA90034]

1/7" twist was needed to stabilize the M-856 Tracer ammo. The original M-196 Tracer would trace up to 500 yards and would still work with the 1-12" twist in the M-16A1.

With the adoption of the new M-16A2, the Military wanted a tracer that traced out to 875 yards to go along with the M-855 ammo. In order to trace to 875 yards, the M-856 Tracer was a much longer projectile to hold the tracing compound.

The M-16A2 needed the 1/7" twist to stabilize the M-856 Tracer.

^^^^^^^^^^
This is the same information that I had previously learned.
It was specific to the needs of M856.

[smle-man]

The M856 doesn't really light up until it's nearly 100 yards downrange so that it lasts longer. That's in the daylight. At night it looks like it lights right out of the barrel. What was I doing shooting tracers at night? I had an inquiring mind that wanted to know. So I tested them out when it was not quite day and not quite night.

Tracers don't ignite close to the weapon so as to prevent the enemy from pinpointing the origin point...or so I was told.