I've never heard of an AR mag causing failure to eject/extract, bad mags are typically known to double-feed (2 live rounds), fail to feed. Fail to fall out of the magwell etc. After the round is out of the mag and on the bolt, the mag is out of the equation.
Your best bet would be absolutely verify that the gas block hasn't moved and if it hasn't to just replace the gas tube, gas key, extractor and ejector and springs and be done with it. (shouldn't be more than about $40 total)
This would probably cheaper than all the test firing you want to do with the different ammo.
Again with the new key have it staked with a MOACKS with new screws. Use a stronger extractor spring (i.e. Wolff XP) and/or a Crane O-ring, a new ejector and spring. I typically replace the gas tube if it looks at all worn (at the gas key end) when replacing a gas key.
Chief Armorer for Elite Shooting Sports in Manassas VA
Chief Armorer for Corp Arms (FFL 07-08/SOT 02)
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