Might look at the Leupold 1-4X shotgun scope; it's what I have on my primary CAR-15. Works great, can be had from $200 on up depending on the version bought, and works so well that I bought a nicer one than mine for my son for his CAR-15 a couple years ago. (Mine's a VX-1, his is the VX-2.)
I typically leave it on 1X for fastest acquisition, which is actually pretty fast with the combination of Leupold optics, heavy duplex reticule, and non-magnified setting. But the ability to instantly crank it up to 4X (or anywhere in between) makes it good for as far as I'm going to shoot a little carbine like this. I've actually bought a TRS-25 to replace the little leupold, primarily to reduce weight and bulk, since it's almost exclusively a defensive carbine now; but I just like the leupold so well, the TRS-25 (and a TRS-32 both) still sit in a drawer unused.