my "varmint" rifle [url]http://home.bak.rr.com/varmintcong/jpar.html[/url], pretty darned accurate custom AR15 that weighs in at 17 pounds, wears a 6.5-20 Leupold Long Range scope for spotting those hard to see varmints at long distances, the gun has hit a ground squirrel at 400 yards on the 3rd shot after having fired 2 rounds to "dope" wind and bullet drop from a 100 yard zero.
Plenty of accuracy in the 223Rem AR15 design provided you get a good upper like: MetalCraft Specialties, JP Enterprises, Compass Lake, or one of the other many quality builders.
My custom AR15 does 1/2 MOA or less for 5 shots when I'm using handloads and with factory loads it is usually shooting no worse than 3/4 MOA for 5 shots. Recently I started shooting 10 shot groups with my handloads and they are holding 3/4 MOA. At 400 yards on the steel plates I can spot my hits through the 20x scope and I'm still keeping them nice and tight and totally accurate enough to make consistent hits on squirrels at that distance if you the shooter are good enough.
And about the scope, many people say that a 6.5-20x is too much scope for a AR15 or a 223Rem, most of them haven't been out in a field staring at a distant rock 400 freaking yards out there and can't tell even at 20x whether it's a squirrel or a rock until the bloody thing decides to move or you decide to shoot it just to see if you're eyes are playing tricks on you. For me the 6.5-20 has served well, it might be too much scope for others but not for me. Only in really hot weather where the mirage is killing the scope do I ever go below 12x so as to tame the massive banding in the scope, in Arizona I'd imagine you'd have similar problems as I have in the summer here where in my town it gets up to 100+ degrees between June and August.
I'm gonna be building my AR10 into a nearly identical varmint rifle only it will be chambered in 243Winchester and the scope used will likely be a Leupold Long Range 8.5-25 for really long range shooting on Coyotes and squirrels.
The AR design is great for varmint shooting as there is so little recoil that it's extremely easy to watch your bullets impacting the target. On my custom gun the scope just jutters ever so slightly due to "buffer slap", when in a good prone position shooting at 100 yards the crosshairs will bounce off target about an inch if that, double taps at 100-400 yards are fun as hell. With my rifle there is absolutely no rearward recoil force to jar you off target, it's like a freaking video game for the most part.