Hasn't been released yet. Your problem with 300blk nose dives in 223 AICS mags is usually due to the COAL. People have made the metal AICS 223 mags like 80-90% reliable by playing around with the COAL but I don't think it'll ever be 100%. Some say the polymer MDT 223 AICS mag will work with 300blk but the polymer mag isn't compatible with all chassis and fitting may be required.
Crisis averted. I think it figured it out. I failed to mention they were new. Instead of just loading a few rounds, I topped it off and cycled the crap out of them until they ran smoothly. I'm pretty sure the key is to keep them topped off so the rounds feed evenly.