If you can reliably hit a man-sized target at 300 yds, particularly with 3 shots in 10 seconds, then you are OK, and a decent rifleman.
If you can do that while standing, you are a studly man.
If you can do that while moving, and taking fire, you are an actor in a movie.
Optics help, but won’t be the differentiator with an AR at extreme range.
NET: get what you like and feel ‘right’ with.
While the .223/5.56 in a standard AR or carbine platform can be ‘accurate’ to 300+ yds., why try that hard? 7.62 and up are better rounds, generally with longer barrels and the right twist.
Besides, that justifies buying another rifle!
The upside of 5.56 is $.40 +/- per decent round downrange. 7.62 that’ll hit what you aim at (unless you reload) is pushing $1/rnd.
the AR15 in 5.56 mm is an extremely fun gun, especially for folks who rarely get to shoot at a range past 200 yards and it is very very capable of hitting man sized targets at 300 yards even with crappy mil surp ammo. With quality reloads or match grade ammo and the right scope, hitting prairie dog sized objects at 300 yards is very doable.
I shot a dime sized 10 shot group at 60 yards with my stag model 8 last week in fact which had a pretty crappy trigger on it at that time. I just installed a hiperfire 24c trigger on it that should have me shooting dime sized groups at 100 yards. I'll find out next week.
I like my .30 cal rifles but, in terms of fun, it's hard to beat the AR15 in 5.56mm.