Gee, I'm not sure what to tell ya. I never really got aquanted with a Sharps. I shoot a Smith and it's had a similar problem.
Obviously I'd suggest taking the nipple out and soaking it in WD40, or whatever you like to clean your black powder rifles with, and do the same with the fire chamber. Then try forcing air through the entire system, from the breech to the nipple. Although I'm not sure you can do that with a sharps, hmmmm....
If you close the action, put a cap on the nipple, point the muzzle at the ground, and snap the cap, do you see blades of grass move or dirt jump from the cap combustion? If you have a clear chamber you should, if you don't I'd imagine you'd get substantial cap blow back.
I wonder if you just need a better nipple? Sometimes nipples have too small or too large of a flash hole, or the taper of the cone in the nipple isn't right for the gun. Try another nipple.
Also, what kind of powder charge are you putting in that thing to reenact? Is the powder inside some combustable cartridge? |