You are absolutely right. Bad judgment on my part, and a measure of my frustration after two days of this crap.
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I think the best way to test this is to swap springs with another lower of similar type (but without the sear block) and try shooting both of them with mags that failed. See if I can recreate the malf in either.
I'll bring along a new spring if the spring from the original lower seems to be the culprit.
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Just a guess...
Since you are interrupting firing a mag, you have manually lock the BCG back. You may not be pulling the CH far enough to the rear and the Bolt Catch could be catching on the Bolt Carrier body and not on the Bolt face.
When you insert a new mag, the Bolt is not behind the next round in the mag but on top of it? Friction of the Bolt or the BC Body could be dragging the top round out of the mag but since the Bolt is not behind the round, it can't make it to the chamber.
Do the rounds that fail to load have scratches/gouges on them from the Bolt or BCG?
Do you see any wear spots in the RE that could hinder the BCG from going 100% rearward?
Just a guess...
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I've checked the mags out in other carbine systems, without a hitch. I'll be working on the springs later this week.