It’s different burn rate powders and pressure curves for the 6.5 vs the 300.
The 300 was BUILT for short barrels, fast powder burn, etc. Its bore to case ratio is vastly different from the 6.5. Even IF you use a 20” barrel and have equal velocity/bullet weight, the Grendel is most likely louder and has more muzzle blast. Now, you could cook up some short barrel loads for the Grendel, to mitigate, but you’d likely give up performance, due to bore/case physics. Bore to case ratio is a big deal for blast/noise under 20”, IME. My 357 max will run 180s to well past 2400 in 16”, with less noise (and often FELT recoil) than my 6.5/6.8 ARs, and even my 5.56 versions ....at least for noise.
Just put muffs on them and roll, but don’t think the Grendel is somehow ‘softer’ on the shooter than the 300. They’ll both have light recoil. The Grendel will likely have more blast, no matter what you do, short of a suppressor.
Something tells me you were right. My son was introduced to the Grendel and the 300 BLK last night. The shorter barrel on the 300 BLK makes for a much more comfortable balance point for him. Then we are down to the CZ 527 and a youth handy rifle in single shot as options in the stable. Again due to weight and balance I think the little handy with a straight 4 power scope may work best at this time. More exposure or testing to come.
Then we will start out practicing with some 123 grain low recoil sub sonic loads. Moving on to hunting loads If accuracy is found with the 110 gr barnes Tac-Tx load we may have found our recipe. We will see how the practice session works out.
Honestly I was leaning to the 24" barreled Grendel though at this point the 16 blackout may get the nod due to his ability to handle the lighter rifle. We will see if he chooses the simple handy or slightly heavier CZ527 both in 300 BLK. Oh that is right the wand chooses the wizard
Note: He prefers to call the 300 blackout by its original name 300 whisper
