The Kaw Valley Precision Pistol Stock Completion Kit that I would recommend for your 10.5" AR is the 4.3oz model. However, if you ever plan on using a suppressor, it would be better to use the heavier 5.4oz buffer.
The Kaw Valley Precision Pistol Stock Completion Kit that I would recommend for a 300 Blackout build is the 5.4 oz (MPN:KVP-PistolKit-5.4) kit. This will help reduce felt recoil and provide better functionality and reliability.