New to BP shooting with a gun that has ignition issue or me?

[D Yager]

Something no one seems to have mentioned in diagnosing this problem. When you pop a cap does it go thunk or is it a sharp sound like firing a .22 blank? The duller thunk sound indicates a clear channel and a sharp crack shows a plugged nipple or channel going into the the barrel. I shoot T7 quite a bit in a GPR 50. Never had a problem with the factory nipple or getting it to light off T7. Really sounds like an obstruction in there somewhere.