Shorter AR15 rifles typically work better with heavier buffers.  I assume the same would be true with 300 BLK AR as a 5.56 AR.

For an over gassed rifle I picked the heaviest buffer and heaviest spring that would still allow the BCG to go back far enough to properly strip the next round and also allow last round bolt hold open to work properly.  In my over gassed Ruger MPR AR15 I found that I could use the heavier Sprinco red buffer spring and an H3 buffer unless I was shooting weak ammo like PMC Bronze.  I had to switch to an H2 buffer with the PMC bronze to get the last round bolt hold open to work.  I don't plan to shoot that ammo very often in that rifle if ever so I plan to run an H3 in that rifle unless in a SHTF scenario where I would switch it to one of my H2 buffers.  That rifle is not the list of my top 5 or 10 self-defense firearms as I have others. If it were then I'd stick with an H2 buffer.  

So many also go with a heavier buffer spring in addition to a heavier buffer weight.  I also have some Sprinco blue buffer springs that are heavier than a standard buffer spring but not as heavy as a red spring.  We may end up with extra spare parts just so they can try different combinations of buffers and springs to properly tune our rifles with their weakest ammo we may shoot.