I'm new here so be gentle. I have a 1917 Enfield that will not close on a field gauge, One of the locking lugs (the left one when the bolt is open), has some pitting on the bearing surface and I seriously doubt it is bearing. I have had a number of split cases, all separating on the same side of the case, going left to right across the width of the case. I suspect the bolt is twisting and allowing the case to separate on that side. I plan to buy another bolt body, check the headspacing, and if it's acceptable I'll try firing it again (off a sawhorse), and maybe lap the lugs a bit. The question is: if the new bolt body doesn't fix the problem or won't headspace, I'm thinking about lapping the lugs and setting the barrel back a thread and rechambering. The threads are square, I don't have the threading bit, and I'm a novice. I'm wondering if I turn down the shoulder to minor thread diameter and back the distance of one thread if that would be a bad way to do it. I think some rifle barrels are not threaded all the way back to the shoulder? My plan is to work it back until the witness marks line up, then deal with the cone and extractor groove. I have a south bend 9" lathe model C. Would a finish chamber reamer suffice instead of a roughing reamer if I've only set back one thread? It would be about 0.092" I think short chambered barrels by Criterion for the P17 are 0.010.