Ruger 10/22
A rimfire .22 autoloading rifle or carbine first introduced by Sturm, Ruger & Co. in 1964. In stock form it's fed by a flush-mounting 10 round rotary magazine, hence the "10" part of the name. It's easily the most popular auto. .22 rifle ever manufactured.
The Ruger 10/22 is likely the most widely customizable rifle of any configuration/caliber on the market today.