Not many 3 1/4" NE were built.

Ammo is more readily available for 3"
especialy if you lost your ammo and had to
get some in a hurry, 3" is much more likely
to be available.

25fps - you get that is shot to shot variation anyway.

100fps - remember is yu are using it in a double rifle
it is going to shoot what it is regulated for.

500 Nitro