What decided short barrel twist??

[P95Carry]

Ok .. let's say sub 3" ... main emphasis on 2" and 2 1/2" perhaps. This will embrace such as SP-101 and similar ...... and sub compact auto's.

This barrel length is barely enough to get a useful powder burn, let alone initiate a good spin ... OK, we are thinkin short ranges I admit but ... still need that gyro stabilization.

Looking at my SP - twist is well apparent but .. how do ''they'' decide what works best?? And - who has figures for these short tubes?

 

[Jim Watson]

The bullet is turning at the twist rate as soon as it enters the barrel fully. Fired bullets show very little "skid" as the rifling bites in. So the stabilization depends only on the twist rate and the velocity, not the barrel length.

I think Ruger uses the old S&W .38 twist of 18 3/4" chosen about 1900 to stabilize a 158 grain bullet propelled by black powder. A faster twist would coke up sooner. Colt .38s are 14" twist.

 

[Vern Humphrey]

Barrel lenght doesn't affect the rate of twist. The key factor is the length of the projectile in calibers.

The classic Greener forumla is typically used, especially with more traditional bullets, like those used in handguns.

 

[P95Carry]

Thx guys ... you cleared up a slight misapprehension I obviously had.

I have recovered bullets successfully from snub shots and true enough ... the rifling is beautifully engraved with no skidding at all.

 

[Standing Wolf]

I think Ruger uses the old S&W .38 twist of 18 3/4" chosen about 1900 to stabilize a 158 grain bullet propelled by black powder. A faster twist would coke up sooner. Colt .38s are 14" twist.

Yep, and now you know one of the reasons for the Python's supposedly "legendary" accuracy: nothing more difficult than better twist.

 

[Jim Watson]

Which is why I shot a Python for PPC, in the pre-L frame days. Also the 7 pound DA from Tedford at the Custom shop. Smith DA jobs are better now, and accuracy plenty good, in a less expensive gun, but then the Python was the class act.

There was also a report in American Rifleman once that a Detective Special would expand a hollowpoint more than a Chief's Special, too.

On the other hand, you can screw it down too tight. Ed Harris once had 9mm BHP barrels made for him by Barsto in their usual 16" twist and from a .38 Special 18.75" blank. Both were more accurate than the military 10" but that could have been better quality and fitting. However, the 18.75" was more accurate than the 16" with the usual run of 115 and 124 grain 9mm bullets.