Gunsmithing Stability effected by less than 1 turn of twist?

[Pogo57]

Subject came up when discussing AR Pistols...

We all know that bullet stability is effected by the twist of a barrel but does how many turns it makes effect it too?

Suppose a barrel has a 1:7 but is only 6.5" long. How stable would it be? What if the barrel was slightly longer than 7" or 14", etc.?

If there is an effect, how close would it show up?

TIA

 

[border_reiver]

Re: Stability effected by less than 1 turn of twist?

When the bullet engages the rifling, the angle the lands are twisting will determine the immediate rpm upon exit of muzzle, with progressive twist the exit rpm will depend on the last couple inches twist rate.. so even at 6.5" long the barrel will spin at an rpm derived from twist relative to velocity. If the barrel is longer the bullet will likely spin at a higher rpm due to a higher muzzle exit velocity. It's a direct relationship there. Doesn't really matter if it makes a complete revolution, look at muzzle loaders with 54" twists, they still generate the twist necessary to spin that ball to accurate returns. 25ACP often only have barrels with about 2-3 inches of rifling, they get spun up just fine.

JR