A rifle buffer with carbine gas in 300 Blackout can be a problem. It will depend on the load and the gas port size. Subs will probably be a complete no.
Most people that use a rifle stock use a spacer so they can use a carbine, 3oz buffer, or H2 4.6oz.
Carbine gas and subs will work best with a carbine buffer.
This is what I was going to suggest. You'll need to use a carbine spring and buffer.
I had to use a spacer in my 9mm setup due to the long buffer tube that I used on the gun.
And as has been said, it shouldn't have a problem cycling and locking the bolt back with 223 either. We used to do that all the time during the ban years because we had to use fixed stocks, which were A2's and Ace skeleton stocks, so there should be no problem with it at all with 223 or 556.
But with blackout subs, the rifle buffer and spring may be a little too much for it.
So, I have a question for you, that is not meant to sound condescending.
If you assembled the upper, when you install the gas block, are you seating it all the way back to the shoulder on the barrel firmly?
Because if you are, you did it wrong.
There is supposed to be a slight gap where the front handguard cap is supposed to go. Lots of guys make that mistake.
I actually made a shim from hack sawing a front handguard cap straight down the middle of it to insert in place until I had the gas block clamped down then I just pull the shim out and I have never had an under-gassed gun. At least not due to a misaligned gas block anyway.
You should also check to make sure that there are no issues with the bolt stop too. Rubbing. Spring tension. etc.