Believe the .348 factory stuff was held to 'under' or around 40,000CUP. Now, what that is in PSI, I don't know.

The Marlins are loaded to 50,000 in the .375Win. and .356Win., but held to 43,000PSI for the large rimmed cases.

The M71 is every bit as strong as a Marlin, I think.

The standard .303, as loaded today is closer to 45,000CUP.

I would think standard speeds for your rifle should be OK, but I'd rather it was on a #4.

Your rifle is beautiful for an Enfield sporter - hope I can emulate it with my #4 someday.