Re: NEW PROJECT - Need Some Guidance - SBR Bolt Action
BUMP for any updates.
I've been tirelessly researching SBR 308's for some time now as well to fill a niche role with my agency. A few things we've found that may be of interest to others here who might be in the same boat.
Rotational velocity will definitely play a role as you get down into the shorter barrels, at least in the case of the 308. Todd Hodnett reportedly took a 12.5" barrelled McMillan 308 out to one mile, but that particular rifle was a bolt gun with a 1 in 8" bore twist.
Typical 16" barrel muzzle velocities for a specified 150gr 308 load we were looking at are approximately 2,600 - 2,650 FPS. We were toying with the idea of having some custom loads tailored for us to drive that 150gr projectile at the same velocity from a 13" piston gun using improved / faster burning propellants.
The problem we ran into is that in theory, regardless of barrel length, if you drive the same projectile at the same velocity, it will exhibit the same terminal performance. However, this is heavily dependant on projectile type. There are projectiles which, when spun too fast, come apart in flight due to centrifugal force overpowering their specific design parameters. Then there are others that won’t open up unless a certain speed / twist has been reached.
For example, if you drive a 150 gr Gold Dot at 2,600 FPS from a 16" barrel with a 1/11" twist rate, then drive it at the same velocity from a 13" barrel with a 1/11" twist rate, it will exhibit the same terminal performance from the 13" gun as the 16". It is not affected by the above issues due to the design of the projectile. However, other rounds may be much more susceptable to problems associated with not reaching proper rotational velocity. The only way to find out would be chrono and ballistic gelatin testing, of which the latter is not exactly cheap to do.
We were advised that one way to help alleviate rotational velocity issues in shorter barrels would be to use a faster 1/10" twist rate. Keep in mind, this was in the case of driving projectiles at the same velocities normally seen in 16" guns. I do think short 308's have a lot of potential, but will take a great deal of tweaking and testing to get there.
Anyway, my apologies for the long winded post. I figured it would be appropriate to address terminal performance issues since the OP mentioned the gun would be used in a work capacity. I hope I was able to provide some useful info to those who may also be looking at shorter 308's.
Have a great weekend guys.
Tspeis