These cartridges aren't anything like each other:
300 BLK subs are basically a 45 ACP round with a much higher BC that gives 45 ACP velocity for several hundred yards. Supers are basically 7.62x39, (7.62x39 is a terrible choice for this application due to bolt lugs failing. Super fun gun but...
Length of pull is definitely big.
I tried starting my daughter off on a Ruger 10/22 with a scope, but then I got her an AR pistol with a 9" barrel and a Trijicon MRO.
I SBR'd the AR, made a form 1 can for it and put a Kahles K16i on it. It's a great little rifle that's grown with her from...
Just came in to post that twist rates are given in "Rotations" per "Length of Barrel". A "12 twist" is slower than a "10 twist". The "10 twist" will rotate 1 & 1/6th times in 12 inches compared to once for a "12 twist".
Hope that helps.
I'd wait to manipulate your gassing on a gun with a cartridge that is intended to be run suppressed 100% of the time until your can arrives. You might have a perfect setup already and be dooming yourself to further struggle.
Non-BT models have turrets that will go stop to stop 43" @ 100 yds for elevation. (43x4=172 clicks)
With the turret apart using the tool that scope will do the same, but with the turret together you get 53 1/4" @ 100 yds clicks for 13.25" @ 100 yds.
Make sure you use "SMOA" in your calculator...