Picked up a used 7400 in .06 caliber. Am waiting for Remington is send me an operator manual for the gun. There is a evident gas leak when fired. Smoke comes from between the barrel forend area. Unit will still eject spent round and chamber another. ( Well, not always chamber a round) But a good cleaning should help. Any suggestions what I should look for to correct the gas leak? I'm about ready to take the forend off without the manual but would like opinions. Thanks. DG