question about ammo and twist rate

[axel2078]

The M&P 15 Sport has a 1:8 twist rate. What is the ideal grain size for this? I'm a newbie to AR's, so I don't know much about this. I found this while browsing on the internet, but I don't know how accurate it is. Can someone weigh in on this?

Rifle Twist Rates

.223 / 5.56mm
1:14 up to 55 grains
1:12 55 – 63 grains
1:9 63 – 70 grains
1:8 70 grains or more

Source: http://www.cactustactical.com/reloading/tips2.doc

 

[MichiganScott]

All lines in that chart should read, "up to". I have a 1:7 match rifle that eats anything I feed it, including 45gr bullets.

Some 1:8 will stabilize 75-77gr bullets, some won't.

 

[Mack]

Barrel twist is not optimized for weight but rather for length. The reason for the military going with a 1/7 twist rate was because it was needed to stabalize the new M856 tracer round which had to be extra long to include enough tracer compound to trace to 800 meters. The M855 ball round which has a 62 grain bullet does not need the 1/7, the M856 tracer with a much longer 63.7 grain bullet does. As a rule a heavier bullet is longer which adds to the confusion about why a faster twist is required.

 

[358156hp]

To compound the issue, twist rate doesn't mandate any specific bullet weight. 1:8 is fast enough to stabilize almost any commercial bullet, but it's no guarantee that that particular rifle will shoot any specific weight with the greatest accuracy. Most people in my area shoot 55 gr ammo, unless they have a need for specific bullet performance. I have seen guys shooting 62 gr bullets in 1:12 or 1:14 twists, and get really decent accuracy out to 200 yards or so. There are a lot of variables involved.

 

[DWalt]

There are methods of calculating the twist rate required for stability. Google on JBM Ballistics. In general, the longer the bullet, the tighter the rifling twist rate required. But lighter and shorter bullets will also usually work OK in a tight twist barrel. Some interesting stories resulted from the twist selection for the early AR-15/M-16 rifles. THe original rate was 1:14 (I think), which was fine. But it was soon discovered that 55 grain bullets were found to be unstable at very cold temperatures, as the air was more dense. Therefore the rifling twist was tightened to 1:12 to solve that problem.