400NitroExpress
so if you do see a double stamped NFB, it is ONLY proofed to BP level - but ALSO regulated for a different projectile (using nitro powders)from its original BP loading, is that what that proof mark really mean?
The 500BPE had a 340gn PP loading & then went to a 440gn NFB load, I suppose to keep a lot of old guns still going & without being re-regulated again but all within the same pressures limits