I've just aquired 1917 DWM Artillery Model Sn. 5005 h. It looks like it spent the last 80 years in some ones attic. About 60% pitted outside, but very good inside, with an excellent bore. Everything about it says "SHOOTER", but I can't get it to fire when assembled. Disassembled, the sear works fine with a thumbnail push, so I'm sure everything still works, but put together, it's a no go. I replaced the trigger lever, thinking this might be the problem, since it looked worn, but it wasen't. I don't want to do any filing on the trigger bar or the firing pin unless I'm sure I should! Can someone offer some advice, please?