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Literally no difference. The difference will be between barrel idiosyncrasies. Some barrels are faster than others, some barrels take a couple hundred rounds to foul up and then will jump in velocity.

If I built a bolt gun today in 223, I want a 1:7, maybe even a 1:6.5 twist. I wouldn't mind having a play with some of the 90-95gr bullets.

I'm currently running a 17" barrel on my GBC straight...

In my personal experiance, a 14.5" barrel with a book max load under a 55gr projectile will yield approx 2800fps. A 16" with the same load will gain 50fps. A 20" barrel will gaim another 50-100fps.