Thanks for the added info. If you're hearing a faint hiss from the exhaust port and it's much easier to turn over than your .40, you're on the virge of needing a rebuild.

My RB X12 has a similar feel (but with only half as much force required to turn it over) as my OS .40 FP (I know, I know, I should have paid the little extra for the OS max but my local shop only had the FP in stock).

Pretty soon, your 12 will not want to idle once it's up to temp. You should start shopping around for a rebuild kit and pick one up when you find the best price. You should replace the piston, sleeve, connecting rod, and wrist pin. You should also get some extra wrist pin clips if you are assembling the rod to the piston (try to get the pre-assembled complete rebuild kit if at all possible). When it starts to get finicky at an idle, strip down the engine, inspect the bearings, and throw in the rebuild kit.

Good luck.