Sight unseen, I think I would opt for the ACS as well, though this presumes that something like a Sully stock wouldn't actually be better-suited to our stated purposes.

The grip could be a standard A2 or a MIAD -- or better yet, a simple MOE -- but durability concerns would seem to rule out most of the Ergo variants.

Handguard furniture is dependent upon whether or not we went mono/polyithic, whether or not we need a free float capability, and whether or not we intend to hang a lot of superfluous crap from the forend. I'm still thinking MRP/VIS, but if not, I suppose Daniel Defense is an obvious contender. That leaves us with about as many choices as Jelly Belly has flavors of jelly beans, but the Omega seems a decent baseline here; especially if the constant battering results in damage that may ultimately require replacement in the field. I did mention that I would favor mono/polyithic, right? LOL

Can't really speak much to internals, though the Y/M National Match bolt carrier seems to be worthy of closer examination. Also, as a southpaw, I'd probably give serious consideration to a Badger Ordnance ambidextrous charging handle, though I presume the majority here would espouse the idea that a Gas Buster is a more on-point upgrade for a gun such as this.

Since this is an imaginary project to this point, let's presume it should have supressor capability. That would lead me to AAC, and to either the SPR-M4 or M4-2000, depending upon the application. I'm not immediately familiar with the durability characteristics of one versus the other. Either would mandate the use of a Blackout variant, of course.

AC