If you release the drive tension you should be able to pull the wire through the gun from the front fairly easily. If you can't then the spool nut is too tight or the liner is shot. Sometimes liners go bad from pinching the gun cable, dropping a piece of iron on it or just not changing a s often as needed. If the liner is bad, get the next size bigger liner than the wire you are wanting to run. Those Lincoln's were notorious for the spool nut tightening while welding.