With the 450/400 3 1/4" , the first thing you need do is make sure the rifle is NITRO PROOFED! The 450/400 3 1/4" was offered in black powder first, and when the NITRO came out, the case was not changed, so a NITRO will chamber, and shoot in a black powder rifle. The 450/400 3" was never offered in blackpowder form, so there is no worry with it. The 450/400 3" will not chamber in the 450/400 3 1/4" chamber, as the cases are different, but the 3 1/4" NITRO version will!

If this is a NITRO rifle it will be stated on the bottom of the barrel flats, and will usually give you the regulation distance as well as, whether it was regulated/proofed for full Nitro loads, or Tropical loads. The full load will be 60.0 grs Cordite, with a 400 gr bullet, @ 2125 fps, 16.00 tons chamber pressure, developeing 4,010 FPE at muzzle, and is one of the best Cape Buffalo double rifle rounds ever made! With proper soft points it can't be beaten for lion, even in close quarters. The rifles are light enough so they can be carried for hours without undue fatigue.

Good luck