According to my copy of the 1989 Rules of Proof book. For Standard Proof 4 & 8 bores are Proofed at 800 bars. 10, 12 &14 bores at 850 bars, 16 bores at 900 bars and 20, 24, 28, 32 and .410 bore are proofed at 950 bars. The larger the bore the less the pressure as the gas has more room to play in and the tighter the bore the greater the pressure; a bit like squeezing a hose pipe and getting to blow off where it fits the tap. Lagopus.....