i am reading the side of a box of hornady 12 ga. 2 3/4 " sst slugs and it says for a weapon proofed by 1370 bar then under that it says MAX 1050 bar. i have never seen this before what does that mean ??

The weapon was to have been proofed at 1370 Bar which is a unit of pressure roughly 1 atmosphere. Or about 20000 psi

The loads max is 1050 Bar. Roughly 15000 psi