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The first hurdle to overcome on the conversion is to resolve an obvious problem with head-space and the .303 British case incapable of chambering in the .410 shotgun.  The problem being the .410 shot-shell has a rim thickness of .063″ with a 55 degree tapered rim and the .303 British having a .060″ untapered rim.  Despite the fact that the .303 British rim is thinner than the .410’s rim, the .303 British simply cannot fit in a .410 chamber without the taper. One way to resolve the problem is to mill a taper on the underside of British rim to make it similar to the taper of a .410 shot-shell.  Problem is, I don’t have a milling machine – but why not just file a few thousands off the case head of the .303 British so it seats flush with the .410 breech.  Let’s do it.

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