Given the perameters, I would do a Rem 700, in 22-250 5r rifling with 12 twist.
The options for the 700 speak for themselves. You can also get the AR style stock and really custom it to your length of pull, and do the same things for attachments as an AR.
I say the 5 r rifling since it fouls less, and I have had great results with accuracy, and the 12 twist would work well with the 53s (i use them, and have shot lots of PDs out past 500yrds without issue) with a working load, you wouldn't have to trim much, could still get factory ammo in a pinch, and it would require almost no hold over at 400, you won't get that with a heavier bullet. The light, long nose, and really high BC get's you what you are looking for.
The only other things I would even consider, is a AR in 20 tac or Prac It will shoot flat, it's an AR for followups, and would be about as close, with 223 brass (cheap, and tons of it) if there is a shortage again the 35 or 40gr 20cal bullets were always on the shelf, and reloading uses less powder, so you can shoot more.
Tough choice no matter what you get.
