My Lee "155s" weigh 160gr w/GC and lube. 15.8gr H110 seemed to be the most accurate with that powder in my 16" barrels with ragged hole groups at 50 yards and maybe 1.5moa at 100 (8" twist AR platform). I didn't chrono 15gr 4227, but it was accurate in my 10" pistol, and still looked good in my 16" despite crimping in the crimp groove (necessary to chamber in the pistol). I know another guy running 16gr or more of 4227 with no pressure issues. Expect to see some pressure signs (like ejector swipes) when running 17+gr MP300. I never pushed 4227 or H110 to see where the limits are, but 17.5gr MP300 will swell primer pockets. I use RP 7-1/2s exclusively.
Realize that my accuracy is a relative thing in that I'm running low magnification optics AND functioning in SA mode with M4 ramps. I see no need for more than 6x, usually less, on an AR. Red Dots can be accurate IF your aiming point is a bit larger than your dot so as to have concentric circles.
I don't have a 300BLK boltgun or SS, but think they would be good platforms to experiment with more bullet weights and different powders. I have listened to 300BLKs (subs)from BAs with cans, and the bullet impact makes more noise than the muzzle report.