A 9mm ar pistol is hard to beat for plinking, even for realistic tactical training etc. The only reason I would go 300blk is for supressed hunting, even then you run into expansion problems with some ammo since they are high velocity bullets in a lower velocity cartridge.
Edit: not trying to bash 300 in anyway just explaining my thoughts on going with 9mm. I am a fan of most calibers including 300.
9mm is as cheap as it gets beside 22lr, doesn't kick at all in an AR pistol, feeds and functions 100% for me with my stern defense adapter and all the dedicated lowers I have seen, and is just too much fun with a reflex or red dot sight! I call it my adult BB gun because I get the same kind of feeling shooting it as I did a when I was a kid shooting cans with a BB gun. Just a kid smacking steel for $.16 a round!
It would also be one heck of a home defense gun
Edit: not trying to bash 300 in anyway just explaining my thoughts on going with 9mm. I am a fan of most calibers including 300.
9mm is as cheap as it gets beside 22lr, doesn't kick at all in an AR pistol, feeds and functions 100% for me with my stern defense adapter and all the dedicated lowers I have seen, and is just too much fun with a reflex or red dot sight! I call it my adult BB gun because I get the same kind of feeling shooting it as I did a when I was a kid shooting cans with a BB gun. Just a kid smacking steel for $.16 a round!
It would also be one heck of a home defense gun