Weaver (the company) orignally developed the cross-slot dovetail "Weaver" rail and the cross-bolt "Weaver" rings to mount conventional scopes only.  The cross-slots on the rail(s) were added wherever it seemed convenient, so the distance between slots would vary between different brands and models of rifles.  Not a big deal with conventional scopes.

Then, alternative sight options came along, most originally for military use, with consumer use coming later.  Many of these sights came with a one-piece base that had two or more cross bolts, and would only work on bases with the proper distance between slots.  Clearly, a standard was needed.

Enter the 1913 Picatinny standard.  It is basically a Weaver-type rail, except that all dimensions are standardized.  The slots must be of a given width, depth, and at a given spacing.  So, there are a standard number of evenly-spaced slots per inch of rail, no matter how short or long the rail.  This allows one-piece mounts to be designed that will work on any rifle with a Picatinny mount.  That's why something like a single model of an ARMS mount will work on an AR15, a FAL, an HK, and a PSS.

I've heard nothing of a change to the 1913 specs, and I seriously doubt this is the case.  The reason given makes (mounting heavy NV gear) makes no sense.  Most NV gear uses STANAG rails, which is kind of like the rail used for an AR15 carry handle (think of the rail on the ACOGs).  These are skinny, yet have no problem holding the big NV scopes.  1913 rails are much wider, and have no problem with these sights.

I find it highly unlikely that there would be a change that would render all pre-existing 1913-spec gear obsolete, especially given that the alleged change is relatively minor, but just enough to make things incompatible.

Then again, maybe I should look for a 1:14 twist barrel, so my bullets will tumble and I can shoot to wound, so that my enemies will have to stop shooting at me to attend to the wounded guy.  Oh, and I need some Pop-Rocks to go with my Pepsi...  

-Troy

Quoted: Now I remember why I keep recommending Bushmaster.

Ditto.

-Troy

The addition of a small radius in the spec, while factual, has nothing to do with the rail width or anything else that would make existing devices incompatible.

Simply, those early RRA uppers were out of spec, and many in-spec parts wouldn't fit without modification.  The whole story about a change to the width of the rail is false.

-Troy