The difference between the 4th and 5th Model Burnsides is the guide screw on the right side of the receiver, with the guide screw channel in the right side of the breechblock on the 5th Model. It made rapid working of the action much smoother and less likely to jam. Of all the breechloader percussion-fired CW carbines, the Burnside had the tightest gas seal, with the cartridge itself being the seal. The Sharps had the Conant gas check in the face of the block, but after a number of shots, it might not function as well. I have fired a 5th Model Burnside, and it is a great arm for its period of time.