Hello,  I'm new to the board and the ar-15 platform.  I purchased a  new S&W MP-15 and have been learning a lot about it on here, but I just had one simple question. If I do not have a magazine in the gun when you cock the bolt back should it catch like it does when you have a magazine in it?  When I do not have a magazine in it I have to manually engage the bolt catch. Is that correct?  Thanks a lot in advance.

That is correct, it is spring loaded in the down position.  The follower of an empty magazine pushes it up to catch the bolt.

Thanks for the quick response.