I've had 2 double action revolvers (out of about 50 that I've owned) that had trouble in firing single action. One was a S&W model 57, a double action .41 magnum. I bought it used, and apparently the previous owner had done a DIY "action job" on it. If i cocked it I'd better keep it pointing at the target, or at least downrange, because if I breathed near the trigger or otherwise made any king of a motion, the hammer/sear engagement would slip and the hammer would fall. Just touching the trigger would set it off. I didn't keep that gun very long. The other trouble revolver was one of several Charter Arms Bulldog Pugs I had. They all were perfectly OK except for one. On it, occasionally the hammer would get stuck in the middle of its stroke when I tried to cock it. I had it worked over by a local gunsmith, and it was just fine after that. The 'smith said he removed a burr. If the action is too stiff you can get it lightened, but I don't know if a "hair trigger" situation can be remedied without buying new parts and getting them installed properly.