The pressure is the same in both vessels depending on the ambient temperature. As you say around 850 lb sq in, rising to 1000 lb sq in in hot weather. I normally put the receiving cylinder for my FWB in the freezer before transfer of CO2 as it will fill easier. You may not be able to do that with your 12 grm as it may be the type that needs to be filled in situ in the pistol.

Baz