I inherited a Colt single action 22 revolver with an additional 22 mag cylinder.
The revolver set in my father's vault for over 20 years. I tried firing it today and only had about 2 out of 6 rounds fire. The dent on the rim is very slight.
I tried 3 different makes of 22 LR ammo with the same results.
The hammer hits a firing pin in the frame. There is not a firing pin on the hammer. Thinking there might be gummed oil around the firing, I put a few drops of WD40 on each side of the firing pin. That helped a little. Now 3 out of 6 shots usually fire.
Should I try a different solvent or take the revolver to a gunsmith for dis- assembly and cleaning?
(First make sure the weapon is unloaded and safe, sorry, always have to say that.)
Try some "Break Free" solvent/lubricant. I would stand the revolver on it's barrel end and place some break free around the pin from the hammer strike side and allow it to remain in that position over night.
This will allow the solvent to work down toward the area where the carbons are blown back into the pin from the cylinder side instead of lubricating it the other direction (from the cylinder toward the hammer) which will cause the carbons and build-ups to go deeper into the pin bore area. With the revolver still standing on it's barrel, manipulate the firing pin with a little more break free and see if it loosens up and the grime should work down toward the cylinder area this way instead of deeper into it.
I would also look at taking the cover plate off to see how gummed up the remainder of the actions are in the workings of the revolver.
Otherwise, a complete breakdown may be in order because of the time, build up, and possible rust that may be in there as well as the other mechanical workings of the weapon.
You might also consider taking the cylinder out of the gun, cock it and with a blunt tooth pick or perhaps a broken off Q tip push the firing pin in and out working the breakfree as SaddleSarge suggested using. If all that fails you may have to look at your hammer spring. One other thing is to make sure your hammer isn't sluggish due to gummy oil.