The rifle has a 20 inch barrel, correct?
That's only a 16% gain with a barrel of more than 3 1/2 times the length. I'd be willing to bet that there is a length somewhere in between that would match or exceed the velocity produced by the rifle. My guess is right around 15 inches.
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.