Basic rail question

[Slater]

There's the Rail Adaptor System (RAS) and Rail Interface System (RIS). Are these entirely different designs/brands?

[Uncas47]

Same brand, the numbers on the RIS are reversed, and the rail attaches at the front behind the sight tower. The RIS came first and are getting a little hard to find. The RAS attaches to the barrel nut and in my opinion is a more solid set up for zeroing lasers etc. Both great rails with a history of service.