The difference would be around 150 FPS maybe and would only matter if you wanted to wring out every FPS you could for long distance shooting. Here's an article about barrel length that may help you although this one is using a 6.5 creedmoor with the .308 further down the page.
The difference would be around 150 FPS maybe and would only matter if you wanted to wring out every FPS you could for long distance shooting. Here's an article about barrel length that may help you although this one is using a 6.5 creedmoor.
The .308 does quite well out of a 20" barrel.
However, when you consider the velocities involved and the somewhat pedestrian BC's of the typical bullet weights suitable for the .308, longer is normally better.
24" is a really good length, if you aren't running a suppressor. Once you add the 9" of a can to that, it becomes a bit unwieldy.
I really like 24" on my .308, however, my primary .308 gun wears a 20" barrel because it was too damn long with the can on it.
If you are thinking of competing, go 24", if is is going to be a hunting gun, 20" will do.
I lost 25 fps when i chopped a .308 down to 18 from 22 inches. For me and that rifle it was worth it, I am still able to make hits at 1000 yards pretty reliably. I read on the forums that 25 fps is what i would lose and thats what happened.
If your going to build one I'd go 18" or under with a .298 bore over a std .300 bore an powder in the IMR3031 burn range. Out to 600yds my 16.5" .308 gets it done just fine. It's short, lite an swings fast. I shed all my .308 bolts that were longer an now am shedding the longer gas guns as well.
When I started as an LE Sniper almost 20 years ago, I had a factory rifle with a 26” barrel that shot Hornady 168 gr. AMAX TAP at 2700 fps. About ten years ago, I switched to a custom rifle with a 20” Hart barrel. It shot the same ammo, unsuppressed, at 2650 fps. I just switched to an AI AX308 with an 18” Bartlein barrel about three months ago. It shoots the same ammo, unsuppressed, at 2550 fps. The barrel length certainly has an effect, but some barrels are just faster than others.
I like the 18” length for its handling qualities in close quarters, whether suppressed or unsuppressed. If you’re hand loading for comps, you can tailor your load to the longer barrel lengths, such as 24 or 26 inches, and likely get a little more velocity for improved performance at longer ranges. However, if you’re shooting suppressed, or shooting comps where the longer barrel lengths would be a disadvantage, I’d stick with 20 to 22 inches as the best compromise between handling and velocity, with 20 inches being my favorite length for a general purpose 308.
I’d do 18” if you’re hunting with it. It’s just so much nicer to maneuver. DARPA did extensive research on .308 barrel length while developing the XM-3 and settled on 18.5”. When I cut my factory AAC-SD down to 18” it was still capable out to 1000 yards and none of the animals seemed to notice a difference.
I have 16-24” 308s in every conceivable variation. 18” with a 1/8 twist and 175 SMK accurate load is only 150 FPS (8208 xbr) off the accurate load for the 24”. I like the 18” over the 16” because the weight of the barrel balances the rifle better. It naturally points easier. Plus, 18” and a suppressor is a usuable length.
Velocity loss and barrel length is very powder dependent. When I use Benchmark powder the velocity difference between the 24 and 16 accurate load is 60 FPS. Just my experience.