Barrel for a "distance" AR Build? (5.56)

[jeeves915]

Hey guys:
I'm in the process of putting together a list of components to do a good medium/long range AR as a Birthday/Anniversary/Christmas present to my g/f. I've got on my list the obligatory Magpul PRS stock, 20rnd PMags, MIAD, Lower, good trigger, etc.

I run into problems though when it comes to plotting/planning the upper. She's commented before she wanted a longer ranged weapon, (Her words, "that's what you and your rifle are for, when the zombies get close").

To me, that means an 18" barrel, and free float rails (DD Lite 12-14" ones). However, I don't know WHICH 18" to get. Centurion, Noveske, WOA, Rainier's house brand, etc. I'm at a loss really.

Suggestions from the experts?
-Adam