The PITA with this build is the magazine. Those 7.5 machine gun mags are REALLY thin around the feed lips. And I discovered the hard way that the dimensions aren't exact, so I had to number the followers to ensure proper feeding after re-testing the now-mixed followers at the range. Real PITA.
Even worse PITA, after creating the locking lugs on the front with a weld bead, I then had to bend the feed lips by hand so each 7.5mm magazine would feed the smaller 6.5x55 cases properly. NOT a fun exercise. I warned him that if he drops the magazine on the feed lips, the warranty is void :)
He loads 100gr rounds, which are short than the standard 6.5x55 140gr round. The 199gr rounds are short enough that they will work in .308 mags work (but only these short rounds). I opted to make the gun a "true" 6.5x55 caliber so the longer 140gr rounds work as well. In fact heavier bullets will certainly feed as there's enough room in these longer 7.5mm mags. Those longer magazines ensure "any" 6.5x55 round will feed, not just the short ones.
In retrospect, If I build another one, I will examine .30-06 magazines like a BAR magazine or posibly even other longer-cartridge magazines (none come to mind right now). This custom-fit magazine feed lip stuff is NOT something I wish to repeat. Definitely worth every penny for a more spendy magazine that works!