The Maxim Defense Industries AR-15 SCW Buffer and Spring will work with that AR Pistol if you are using the Maxim Defense SCW Brace.
The buffer spring will not change between calibers, just platforms and carbine vs fixed stocks. The buffer itself can vary based on your particular rifle. The safest would be to go with the standard carbine buffer, but the heavier H buffers will slow the bolt for a better feeling recoil response. With a fully open gas block, most of the time, the H3 can run reliably. This will differ though based on gas port size.
This should work great in a Maxim Defense SCW Stock for .300 BLK. For .300 BLK under 12" barrel, Maxim recommends a H1 buffer, over 12" they recommend a S buffer.
No, this is a proprietary length buffer and spring for the Maxim Defense SCW stock.