I know there are a couple of topics already on twist rate in other calibers, but I'm not sure if they really translate to 300 BLK. Will a 1:8" twist rate with a 10.5" barrel (if that matters) work for both supers and subs? More importantly, will it even shoot subs accurately? This is my first AR build, and I'm hoping that it'll be good to go for self-defense, once it's finished. I just don't want to have to buy another barrel and all that.

Depends on how heavy of subs you plan on.

I personally prefer 1:7.

I had the same conundrum when dreaming the uppers I built, as I want to shoot subs and supers (mostly supers though).  i too went with 1:7.

Although I have run 110gr supers, I have not checked them for keyholeing.  Typically I'm running 145 or 150 gr and  they are stable as are my 220gr subs.

This apples to both my 10.5" and my 16" uppers.

I can hit anything I want with the light bullets but then again with 300 everything is inside 150 yards so even if the bullets are unstable, their accuracy is not too far off.  I do need to shoot some paper and find out.

Ok, I guess it's just a matter of checking out and testing various types of ammunition at the range once the build's finished. I have no problem with that, honestly.
Thanks for the replies!

Also, I'd be interested to know what you find out if you get around to it.

Probably OK in a 10.5 since you are getting a touch extra velocity from the length, thus a higher RPM.   I have not shot subs on paper in a long time, so I can't say much about keyholes.  I'm also in Illinois, so I guess I'm "lucky" enough to not have to worry about baffle strikes.  

Yeah, I was thinking the barrel's length might be beneficial. But man, I didn't even think of how bullets can hit the inside of suppressors. Sucks for your ears that you live in Illinois, though.