Well if velocity was the only concern then for factory calibers ,it would be the 204 Ruger. I have had my 22/6mm imp up around 4700fps with 40gn bullets. I have some 30gn Bergers to try in my next barrel ,just to see what they do over the crono ,if they hold together. Some people have played with light bullets in the 22/284 which is bigger again.

seem to remember something about a 338/50bmg think it was getting high 4's or low 5's.
I know I have read about someone getting 5200 or 5300 with small copper bullet.
I'm sure if you to get a 30cal bullet in a 50 cal sabot and a long enough barrel in the 50bmg that would be it.

I got my .240 Incinerator up to 4,800 fps with a 55 grain ballistic tip (using surplus WC 860 and a 28.5" tube).

Lee Martin
www.singleactions.com

OK Lee I will ask what case is the 240I based on?

I believe it is based on a .264 Win Mag necked to 6mm. 35 degree shoulder and minimal body taper.