Stag 15 Varminter. I have a pair of this rifle's predecessors (
Stag Model 6 Super Varminter), which are no longer available. But the 15 Varminter is essentially the same rifle with a better stock.
The 24" barrel is optimized for the longer distances, and should handle the 75/77gr MK262 equivalent commercial loads to their best advantage. Mine has been fired (by my Granddaughter) in F T/R MR 600yd competition, and handled the course very well. I ended up giving her the rifle, then built my own replacement from the Stag M6 Rifle Kit.
The AR15 will shoot a lot further than 600yd, but under competition conditions, especially wind, I choose to call it a day at that distance. For comp at longer distances, the 6.5 Grendel AR''s, and the 260 Rem Bolt gun are good out to about 1000yd, and 1 Km, respectively.
This scope, in combination with a decent 600yd LRF, will take a lot of the computation/guesswork out of finding Aiming Solutions in 100yd increments out to 600yd. It is calibrated for the 55gr SS-109 thru 62gr M-855 trajectories, so it will be off (shoot a bit high further out) with the MK0262 clones, but practice will simplify that.
I use this line of scopes for several of my bolt guns, as well as for some of my my AR's.
The 15 Varminter is not a run and gun rifle. It's extra long and the bull barrel increases the weight into the beastly class. But for LR and challenging targets, it's the scalpel of AR15's. You can pay more, and you can get better, but as the bare bones git 'er done LR factory rifle, the basic item has been serving my needs for over the past decade.
Like you, I was totally a bolt rifle guy (nearly all of them Savage-based rifles) until I got the Varminter. Now, I have the bolt guns, but I do almost all of my current projects with AR's (15's and a 10) these days. I use the 308 BDC scopes for my AR-10, and for my 6.5 Grendels, and the 6.5 Creedmore BDC for my 260 Rem. They are very close, within 3" at 500yd.
For closer-in run-n-gun, my choice is the Ruger PC Carbine, paired with the Ruger American Pistol 9 Pro. The 17rd magazines interchange in this pair.
Greg