A couple of notes - make sure that you re-tighten the set screw on the side of the hammer to prevent the hammer screw from walking back in. I had that problem over the summer - could not for the life of me figure out why I was loosing velocity... I did wind up figuring that out and make double sure I tighten it back up each time now.
For the SSH, I don't think that the hammer screw needs to be all the way in, so only the inner hammer hits (that was my 550 fps problem). Just that when the outer hammer hits the shoulder, then the inner hammer needs to slip forward a bit, continuing to open the valve via momentum. Then when the valve returns, the inner hammer retracts via its spring and the outer hammer does not move with a heavily preloaded or strong short spring. I might try the original spring again to see what the efficiency is now. One thing is certain, the factory does not "tune" the rifles for efficiency off the production line.
I would agree that 0.95 efficiency is not good. I one time got 1.7 with my Compatto 177, and 1.3 from this rifle - just don't know what the secret formula is to get it back there. One other possibility is that the pressure is decreasing as a result of cooling of the air in the tank as I shoot. I am filling at 65 deg in the basement (tank is really warming to 80 deg-ish via the filling process, which other folks may see as well), then shooting out of my garage at 25 to 30 degrees. Even in my 10M shooting cellar, it is 45 degrees. So maybe some of the "air use" is really from the temperature decrease, hence the efficiency appears poor, but really is "ok". I did some math 80 deg to 30 degree gives about 10% pressure loss - back fudging the pressure numbers to account for temperature swings, indicates that real efficiency might be around 1.22 fpe/ci. I would still like to get it more efficient tough. I am hoping for some warmer weather to shoot outdoors. I also just ordered some of the redesigned JSB 13.43 177 pellets to see if they group better at 55 yards as well.