Thread: 308 twist rate

Originally Posted by Bill999

for short barreled subsonics up to 1-5 work great
1-5 seems to be the standard these days for the machined expanders and works just fine with supersonic
the extra rotational energy is bugger all but it gets the expanders spinning like an outboard propellor inside the target giving you a bit more energy transfer
you can go slower twist if your barrel is longer

1-7 or 1-8 would be great in your application 1-5 is pretty much just for short barrel subsonic dominant guns (which is my current obsession)

Bill999 I just can't agree on your choice of 1:5" for sub and supersonic, and your statement that in a longer barrel you can use a slower twist rate.
A twist rate of 1 in 5 means the bullet turns 1 full rotation per five inches of travel. A 1 in 8 twist means the bullet makes one revolution every eight inches of travel (it has to travel more distance to make a full rotation). But adding length to the barrel ie going from a 16" bbl to a 24" bbl will not make the bullet rotate faster than 1 time per 5 or 1 time per 8 inches of travel (whatever is the barrels twist rate).

The only way a longer barrel could make the bullet rotate faster is that it has more internal space for powder to convert into expanding gas/pressure therefore velocity can increase. But that is mostly theoretical as one generally cannot reach safe pressures to increase the velocity enough to get RPM in a 1 in 8" twist longer barrel equal to a 1 in 5" shorter barrel.

So to summarise: spin is dictated by the twist rate of the rifling, no matter the length of the barrel. 1 twist in 8" is the same from a 16 " barrel as it is from an 18", 20" or even 24" barrel... one complete rotation of the projectile every eight inches. Really the only practical variable that will affect RPM is the rate of twist... not barrel length.