Usually I test the gun first before I modify it to be sure it's not defective, and for a before/after comparo, but sometimes I tear into them first thing too. Oh well.
Did you look the insides over, like take the piston out or anything? Usually cheaper guns need a new main seal because the orig is damaged/cut during factory install. Also many/most have seals that aren't ideal, either poor design and/or poor materials so it's good to replace it.
For some reason I was thinking you had a Ruger mag, but I see it's a Blackhawk. The mags have a picatinny rail which they weld on and the welding has ruined plenty of guns. The weld leaves unfilled pockets inside which break the seal. The first one I saw let the air leak out for the first inch or so of travel so a lot of that air was lost, as a result it was lower powered than the standard gun. Unfortunately many people don't know it because they have no way to test power.
It's hard to estimate power without a crony, but one way is to shoot lighter and lighter pellets until you hear the super sonic crack. Like shaving weight off the head of a light pellet and using a jeweler scale or powder scale to weigh them. Somewhere between 6 and 7gr it should go ss, then calculate the pellet weight to the speed of sound where you shot. Just a thought...

If/when you get the correct seal you might consider doing a little tuning while you're in there. I made a tuning guide that describes it in detail, including pix. Things like sanding the face of the seal down, piston buttons, transfer port work, breech work, scope mods, lube info, lots of stuff... If you want it write me at chevota@hotmail and I'll send it to you. Not only will it make more power when done, but smoother and easier cocking, smoother shooting, better accuracy, less scope abuse, and much longer gun life.