Any .45/70-type ctg. chambered up in a #3, will take any pressure the action can withstand on a regular basis. There is no problem with strength in modern brass. Marlin ran a Marlin lever gun to 70,000psi and it extracted normally, without damamge to brss nor rifle, of course. This was a test only of gun and brass - not a normal usable load. modern brass won't take that sort of pressure, ie: .270, .338 mag. whatever, as the primer pockets will expand. The rim itself gives immense strength to the brass, coupled, of course, with modern heat treatment.

The low pressure SAAMI ahd CIP numbers for the .45/70 are due to the old actions still in use, ie: Springfields and other original black powder rifles not suitable for heavy charges of smokeless powders.

This does not hold over to the modern guns, of which the #3 is one. #4's & #5's are even better in the strength department.

They can be loaded to any pressure the .303 is suitable for in #3's and higher yet in the stronger actions.

Even a #3 will run 400gr. to 2,100fps.