C&R stands for Curio & Relic.
Some rifles, handguns, shotguns, and full auto weapons are listed by the BATF on the C&R list. Those with a C&R License can buy the non FA guns across state lines, in most cases, and have them shipped right to the door. They can also own FA guns in those quaint states that only allow their peasantry to own Curio & Relic full autos.
AR18 a full auto rifle chambered for the .223. It was made so developing countries could build it without a lot of investment in fancy, expensive tooling. The semi auto version is the AR180, and a newly made semi-auto the AR180B is now available. There are some threads about it on TFL.
I shot an AR18 at Gun Fair in Princeton, Minnesota once. Fun, about the same size as the M16, but felt different. I should have started buying one then. I had just finished buying three subguns over a two year period and was broke and suffering the effects of "domestic warming," at the time.