300 plus yards with an AR.....
I can say that I have never shot an AR that wouldn't hit a standard B27 type target (COP type target with a head lump on top, I'm fat but not that fat!) at 300 yards.
To be precise and building for it. I have/would go with a 20 inch barrel (greater velocity potential - optics don't care about long sighting radius), heavy is sometimes better. Good mounts and better optics. 20 inch barrel is standard rifle length gas and buffer. Although I like telescoping stocks, a good A2 ridged seams more solid (to me). GI spec lower and a good/very good trigger. Your choice of single or two stage trigger, but crisp break. Twist is a big factor if you plan on shooting long/heavy bullets, but they start out slower. It is a balancing act with velocity, BC of the bullet, bullet weight, twist.... Good bullets shoot good, better bullets [can] shoot better. Build your loads with care and they will respond.