I'm not sure a slightly heavier bullet would work as a proof. Going from 300 to 330gr. in bullet weight, usually results in only 1 or 1.5gr. powder difference in the .444 top-end loads - from memory here. Most data in various loading manuals varies more than that in itself. Inceasing pressure by 30% seems a bit drastic, but then, we-too have no proof rules here.
: Does anyone know the SAMMI standard for .444 Marlin? Is it the same as all rifle's used in that ctg. as in 44,000CUP for Marlin and WW lever guns? Is the double rifle stronger or less strong than these rifles? What is this level of pressure in PSI? or CIP for that matter?
: I think if one wants a proper proof, one needs to send it away to be done properly where proper pressure equipment is available IMHO.