With 12 ga guns, they do say to stay under about 7,000 psi. When you go down in bore size the pressures will go up. I do not know what the number is in 16 and 20 ga but it should be higher. That is why I just use black powder in my 20 ga Ithaca Flues. I do not know what pressure levels the black is producing but it should be OK because my loads are light.

Jack