Mistakes you only make once

Built a P-14 Enfield rifle with a 1910 Maxim gun barrel in 7.62x54R. The 7.62 x 54R has very thin case walls close to the rim (unlike most rimless cartridges which have an almost hemi type chamber around the primer). The extractor cut is very critical and cannot be too deep in the barrel or the case will fail under 50,000 psi and bad things happen. The first test fire of the gun blew out a nice rectangular slot in the case, blew my glasses off, ruined the extractor and put a few flecks of brass in my face while making my ears ring even with the protectors on. I set the barrel back a thread and re-measured everything very carefully, re-reamed the chamber until headspace was correct and it is a tack driver but LESSON LEARNED!