I had the same issue although my gun is shorter and has a pistol length gas system.
The remedy is more powder. You need a higher charge with the same bullet to get the BCG to go rearward enough to pick up the next round.
Keep in mind that the .300/.221 is much shorter than the .223 so it is easy to eject without a full cycle of the BCG.
Try going in .3 or .5 grain increments until you get reliable function. The only problem is that it may also be supersonic when that happens. That is the problem with the AR platform is that it needs to be stout enough to cycle the action, but slow enough to avoid the sonic crack. You may also have to play with some powders. Mine likes 8.5 grains of 2400 with 220 gr. Sierra Matchkings. I also tried it out and got it to work with lighter bullets (175 and 150) with 7.5 grains of 2400.
Last thing is since you have a carbine gas system, you may need to open the port up if you want subs that cycle.
Since you didn't list your twist, you may have better luck with lighter bullets if it isn't a 1 in 8.
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