Can someone explain to me the difference in the two?  I have an RRA on order and I have been looking for a scope.  A lot talk about weaver rails.  So, is the Picatinny rail wider narrower or what?  I would appreciate it if someone could explain the difference.  I want to buy my scope/optic and have it ready when my gun comes in.

Thanks!

The mil standard 1913 rail is just that. it has standards in dimentioning the gov't supliers are supposed to adhear to in the dovetail, nothches width depth and spacing. If they are on a flat top receiver then they are supposed to be a certain ht. off the center line of the bore. Weaver is a type of design of the same dovtail concept that has no specific dimentions in the way the military rail is called for. Sometimes weavers can look very much the same as the mil std 1913 sometimes called the Picatinny rail, that was conceptually taken from Mr. Weavers dovetail design concept, but the mil standard refined and established it to military needs.
That is a brief descriptive diferance that I hope helps.
Good shootin, Jack

Jack,

It does help.  Thanks so much....

Is the "squared-off" ring notch peculiar to only one of these designs? Or can either have it?

Picatinny rail uses squared shots (and the cross lugs on the mounts are also square) and the rail is slightly wider (side to side) and the lugs thicker (top to bottom).  Many "Weaver" mounts will not mount correctly on M1913 rails because of their more robust size.

-- Chuck