I put a Nikon P-223 on my sons. Simple, fixed 3x , easy to get behind and very compact--about 8 inches long. They are discontinued, but I got his for about $100. Might find a used one....
The P-223 Rifle scope is the perfect addition to any AR-15 and provides extreme target acquisition with the BDC Carbine reticle. Designed specifically...
The UTG bug buster and Hawke vantage are available in 4x, 2-7, and 3-9 power. 32 mm objectives and adjustable parallax for up close. They have red green illumination too.
Started with a little CV life 4x32 scope off of amazon. Good little scope for the money, but lacked magnification. Upgraded to a Nikon Prostaff rimfire 3-9 x 40 and my son loves it. It's a little big, but works great.