I hear Starline is making .348 Win. brass, which is better, Winchester or Starline? Is any one making 35-348 Ackley Improved brass?

Any one have any idea how much velocity I would loose if I turned my Mirkoru Win. Mod. 71 into a 35-348 Improved and cut the barrel from 24" to 22" ? My goal is 250 grain Kodiak Bonded bullets and H4350 at over 2,500 fps. Maybe I am being optimistic on velocity. I know, blow a case out to get more velocity and then cut the barrel back to get less velocity, don't say it.

I would also like to find a magazine extension if any one knows where that can be found. Darn rifle only holds 4 in the magazine and my Marlin 45-70 and Big Bore .356 Win. each hold 7 in the magazine. Having the extra ammo in the magazine is the perfect place to store ammo!