Velocity loss/gain

[nmkid]

So, I've always read that you lose xxxx number of FPS by shortening your barrel. Say (in general) you have a 26" barrel and you cut off 1" and lost 40FPS. If you added an inch would the velocity gain be the same number? Not really getting at anything here just sitting here bored waiting for the final piece for my current build and it's 2 weeks overdue!

 

[dcali]

More or less, yes. Not exactly, but the numbers are so fuzzy that you can basically assume that within reason. If you're bored, buy yourself a copy of QuickLoad, and you can run all manner of combinations to get a feel for this sort of thing.

 

[Will Henry]

Yesterday, I was shooting two 308's, using the same load (4320 and 155 Scenars). The chambers are identical (same reamer). One barrel is a 26 inch Douglas, 12 twist; the other is a 29 inch Krieger, 13 twist. The longer barrel averaged 17 fps faster. Of course, the only definitive test is to shorten the same barrel but, in general, the difference is often less than one might expect. I would expect the difference to be greater with a larger cartridge and slower powder. WH

 

[Ned Ludd]

In my hands, barrels in the 16"-26" range typically show about 15-20 fps increase in velocity per inch of barrel. Some of that comes from shooting the same commercial loads out of a switch barrel .308 rifle I have with 16", 18", and 22" barrels. The rest comes from anecdotal observations made with different rifles, so take it with a large grain of salt. One thing that does seem to be fairly consistent is that the increase in velocity per inch of barrel length goes down substantially for barrels longer than about 26" with commercial ammunition. I suspect that has to do primarily with the powder/primers in the commercial loads not being optimized for longer barrels. However, even with handloads in 28"-30" barreled F-TR rifles, I don't typically see anywhere near a 15-20 fps increase in velocity per inch of barrel. It's much closer to something [well] under 10 fps/inch barrel.