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Old June 6, 2019, 06:58 PM   #11
Senior Member
Join Date: November 21, 2013
Location: New Hampshire
Posts: 1,500
I too had feeding issues with 556 Pmags when shooting 220 SMK or 220 Leatherhead subs. I did some research and the issue is with the reinforcing ribs on the inside of the mags. You need to file about .060" off each rib and this will allow smooth feeding of the fat and heavy bullets. I also bought a dedicated 300BO Pmag but it's a 30 rounder and I can't shoot off a rest with a 30 rounder. (the 300 BO specific pmags don't have the internal ribs). I like 20 round mags for benchrest or bipod.
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