I have 3 rifles chambered for 350 Legend, one using AR style magazines. Came with and ordered additional Duramags. That particular rifle being a turn bolt, I didn’t have last round hold open to worry about but I did have multiple gremlins.
After examining brass, I began with the feed lips which were leaving striations. Disassembled the magazines and gently sanded with 320 grit until the feed lips no longer resembled razor blades to the touch.
Still had feed issues. Rounds were jamming during cycling. I removed my magazine and began stripping cartridges by hand, noting that often the case mouth would contact the front of the magazine. I whittled at the front of the magazine until I had 100% reliable feed both by hand and with the rifle.
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Above showing modified magazine (with round pushed forward to show relation).
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As received factory magazines.
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Side by side.
I am not an expert in all things AR; I have shot several for a decade or more and assembled a few. I do whole heartedly agree that feed ramps can in no way interfere with lock back. Not ever. If all that’s been done to your rifle was a polishing of the ramps, your hold open isn’t fixed.
I would hunt down a second manufacturer’s magazine to test. I would also weigh the buffer just for fun and find out what others are using. Lastly, though first in actual order, would be to make sure the rifle is correctly lubricated. I got so wrapped up in a pistol build once that I neglected to oil anything. Fired 2 rounds, had to mortar both out, then my brain finally came through for me.
Best of luck.