Questions about rifle twist AGAIN

[tnmt_donkey]

couldn't sleep last night, came up with a couple newbie questions about rifle twist.
please forgive me if the questions are too stupid.

what is the rifle twist like on those CQB short barrel weapons?

I know that the standard RT on AR-15 A2 upper 20" is 1/9".
what would happen when the barrel is cut into half? ( same RT, but half length)
would there be a huge difference at about 50 meters?

I am new and learning....
Thank you for all your replies.

 

[strangenh]

Stability is established very quickly, length-wise. A shorter barrel mostly decreases muzzle velocity (and provides a fireball at no extra charge).

The other big difference going from 20 to 10 inches is the federal paperwork and tax stamp, assuming you're talking about mating it with a rifle receiver.

 

[lupis42]

Stability is established very quickly, length-wise. A shorter barrel mostly decreases muzzle velocity (and provides a fireball at no extra charge).

The other big difference going from 20 to 10 inches is the federal paperwork and tax stamp, assuming you're talking about mating it with a rifle receiver.

Weeeeel... sort of.
Stability is a function of projectile length and projectile RPMs (More length requires more RPMs)
A shorter barrel reduces the velocity which means that you need a faster twist to get the same RPMs, so (for example) a 1 in 9 20" barrel will stabilize longer projectiles than a 1 in 9 10" barrel. Most of this is academic for most purposes (i.e. in a carbine, most 5.56 will be stabilized fine by a 1 in 9 whether you have a 10", 14.5", 16", or 20" barrel) but if you want to stabilize very long bullets (e.g. 77gr SMKs) you need a fast twist. Furthermore, since it is possible to over-stabilize, if you want to shoot, say, 40grain varmint loads out of a 20" barrel, a 1 in 7 is not for you. (The bullets basically explode a in the air from the centrifugal stress).

 

[tnmt_donkey]

wow, nice info.

so 10" with 1/9" will still stabilize the 5.56 projectile. it is the muzzle velocity limits its capability of hitting long distance target.

in this case, what is a suppressor is added onto the 10"?
slower the muzzle speed further??

 

[lupis42]

wow, nice info.

so 10" with 1/9" will still stabilize the 5.56 projectile. it is the muzzle velocity limits its capability of hitting long distance target.

in this case, what is a suppressor is added onto the 10"?
slower the muzzle speed further??

A good suppressor will have basically no impact on the projectile.

FYI, there are multiple 5.56 projectiles - M193 will be fine out of the 10" 1 in 9, Mk 262 not so much. It's all a function of length.

 

[strangenh]

Weeeeel... sort of.
Stability is a function of projectile length and projectile RPMs (More length requires more RPMs)
A shorter barrel reduces the velocity which means that you need a faster twist to get the same RPMs, so (for example) a 1 in 9 20" barrel will stabilize longer projectiles than a 1 in 9 10" barrel. Most of this is academic for most purposes (i.e. in a carbine, most 5.56 will be stabilized fine by a 1 in 9 whether you have a 10", 14.5", 16", or 20" barrel) but if you want to stabilize very long bullets (e.g. 77gr SMKs) you need a fast twist. Furthermore, since it is possible to over-stabilize, if you want to shoot, say, 40grain varmint loads out of a 20" barrel, a 1 in 7 is not for you. (The bullets basically explode a in the air from the centrifugal stress).

Agreed on all points. The OP did spec that the twist was to be 1:9 and only the length was changing. He asked for the impact of the length change. That's what I was answering.

 

[Jasper]

Even a 20" 1:9 is marginal at best with a 77gr bullet (like mk262). Wouldnt even bother with a 10"

I still have plans for a MK18mod0 clone (10.5")... but i will be running a 1:7 for that