Hedgemeister, I'm not trying to argue with you, but just playing devil's advocate. Like I said, I've never been to the "sand box" or on patrol, but I do live to play with guns. I'll say this, I don't think there's any such thing as a rifle that will run no matter what.
Yes, there are piston guns (and DI rifles) that come with uber coatings that apply a permanent lube to the parts etc, but these coatings only work when the parts are clean. It does NOT guarantee that the rifle will run no matter what. Lubing gun parts does two things. First it lubes. The second is that it provides a means for the action of the rifle to displace crud/dirt as it builds up. Of course, some lube works and lasts longer than others, but that is a topic for another discussion.
My point is that even a piston gun is going to need maintenance. I don't care if it's piston operated or not. If you're running it dry and a good bit of crud/sand/dirt builds up in the locking lugs (or elsewhere) and there's no way to displace it, it's going to stop running at some point. In a couple of extreme cases, with the owner believing that the rifle didn't need maintenance, I've seen pistons lock up. I'm talking about the op rod refusing to move. The moral of my rant is that both are going to need lube and I don't think that there is any concrete data that would indicate that one system is more reliable than the other, assuming proper care.
I think the important thing is to get her a rifle that fits her and that she can be proficient with. If she is on the petite side, a front heavy rifle may not be the way to go. A front heavy 7lb rifle swings like crap compared to a well balanced 8lb rifle. A well balanced 7lb rifle works like an extension of your body. I don't think that anyone can call any piston gun well balanced. It may not matter to a 180lb guy that trains with the rifle everyday, but for a 125lb lady who only has to qualify with the rifle twice (once?) a year, it may matter.
Since it appears that cost is no object, if a piston gun is engraved in your mind, I'd go LWRC. If you are open to options, I'd be looking at Noveske or Daniel Defense. Noveske and Daniel Defense may not offer anything more than the likes of Bravo Company, in terms of reliability, but having the name may give you piece of mind. It seems to me that a Colt would be the logical choice, since the armorer already works on them and is probably familiar with any quirks (if any) that the Colts may have.
Personally, I don't buy ARs. I build mine, but if I were to buy one, I'd buy it from either Bravo Company or Spike's Tactical and for a couple of reasons. Both build excellent rifles and with both, most problems are handled with a phone call that usually results in replacements being sent out right away.
