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Everyone is saying to use a heavier buffer and sounds like nobody knows how an AR works. But if it isn't cycling subs, you may have a different issue. Only use a heavier buffer if there's too much gas, to slow down the cyclic rate. I don't have one but someone said it's 3oz. That's a carbine buffer. Try a sprinco yellow spring (its a little under powered compared to other springs). Sounds like it's not getting enough gas which could be gas port size, gas block not being aligned or too the spring is too strong or buffer too heavy. Assuming the gas system is aligned correctly, adjust the spring tension.
I have this on my 8.5 aero pistol in 300 blk and it won't cycle subs even after installing an adjustable gas block and opening it all the way.
Need to get a lighter weight spring for subs. Supers cycle just fine with this.
Update, its a 3oz buffer. You most likely will prefer a 5- 5.5 oz buffer. Can buy a rifle buffer or just swap out weights to 3x tungsten alloys. This will work for you and cycle fine but may kick harder than perfered.
Others claim it to be a "carbine" buffer so I'm assuming its h-h2. You may want to up the weights in the buffer to go h3 for blackout. Mines on order so can't confirm.
Anything used for a 5.56 can be used for a 300aac except for the upper barrel assembly. This means bolt carrier group, magazines, buffer tubes and springs are all the same.