I'd check the cylinder pin to make sure it us fully seated so that the set screw is in the barrel indentation. (See BFR manual online or the one that came with the gun). I had a Ruger seize up because the cylinder was sort of cocked and it would hang up on the forcing cone due to the tight tolerance between the cylinder and the forcing cone, base pin issue. Also, check to make sure that the transfer bar isn't catching on the firing pin. Have seen that this can happen on older more broken in BFR's. Worst comes to worst, send it in and have Magnum Research give it the once over, probably not a bad idea since it is a used gun. Let us know if you get it figured out. Good Shooting.