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.
