I've got a czech k98, barrel in good shape but it seems to leak gas around the cartridge mouth. There is usually a bit of black around the case neck. That is with factory ammo. With reloads the other weekend, it started off shooting ok but after about 50 shots or so there was a black line down the side of the case all the way to the rim. What could be the cause of this? Is this a serious safety concern? I've since cleaned the rifle but not fired it. Could this be a headspace issue?