After endless hours fiddling with this, I believe I have a solution.
At the heart of the issue is that in some stocks the magazine sits too low with few realistic solutions to force it up the ~1/16" necessary. As a result, the case rear, when engaged against the block at the back of the magazine, is too low for the bolt to engage it. Watching what was happening with a set of dummy rounds I'd made up, I could see that if the case were set forward from the back of the magazine by as little as 1/16" or so, it'd be high enough on the follower for the bolt to engage it. I experimented with a number of approaches and think I found a simple solution.
Two thicknesses of 3M 2242 Linerless Electrical Rubber Tape (basically really thick electrical tape, URL: http://solutions.3m.com/wps/portal/3...4920609&rt=rud) trimmed to the correct width and applied to the face of the rear block provided the necessary forward space.
I ran into some issues because of the 'grabbiness' of the tape surface. This was solved by placing a strip of Lee Valley Low Friction Tape (basically a really thin strip of UHMW, URL: http://www.leevalley.com/en/wood/pag...10,43466,32182), which kept the cases sliding smoothly against the shim.
I've cycled 30+ magazines worth of dummy rounds through testing this tonight without a single feed failure. Prior to this, the bolt would override the second last round three out of four times, and nine times out of ten with the last round.
I'll need to field test this at the range of course, but we could be looking at a universal fix here.
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