Picatinny Rail vs Weaver base: Whats the difference?

Hello all!  I hope this post finds all well and enjoying the post Thanksgiving Food coma days and the hustle and bustle of getting ready for Christmas.

Over the holidays I had a couple of gun related conversations and one I thought I would bring to light is the difference between a Weaver base and a Picatinny Rail.

The above picture is of a Bushmaster XM-15.  To the left is a Picatinny rail which is built into the XM-15 for mounting a sight, carrying handle or anything else one deems necessary for their weapon. On the right is a weaver base hand guard.

I first saw a picatinny rail system in the early 90s when the I was introduced to the M-24 Sniper Weapon System(SWS).  The M-24 utilized a Leupold Mark III scope and you tightened the scope to 75 ft pounds using a torque wrench to the base.  Where as the M-21 SWS utilized the Art II scope which had a side mounted scope bracket that you tightened with a handle.  Here at this point was the first time I heard the interchanging of Picatinny rail and Weaver base.  Then later in the 90s the Army adopted the M-4 and the Special Operations Product MODification (SOPMOD) and just like blue tooth rail systems were every where.

So what is the difference?  First off the design for the Picatinny Rail system comes from Picatinny Arsenal in New Jersey.  ( I gotta ask if the designer is kicking himself for not going public with it first)  It is Milspec which really means nothing except that is governed by regulations.  The Weaver base is civilian made and to less stringent machining specifications.  To the naked eye spacing, width, and thickness appear to be the same.  However if we take a micrometer to the base/ rail we will find that there is a difference between the spacing of the  Risers.  The Picatinny has a set width for the groove which is .206 and a set center to center width of .394.  Where as the weaver has a set groove width of .180 and no set distance for center to center.

Here is where the buyer beware comes into effect.  With out the above knowledge in hand I bought a laser with a mount for the Picatinny rail.  I then attempted to put it on my Weaver Base hand guard and it would not work.  However when I tried it on the Picatinny Rail section built on the weapon  (left in Picture) it did fit.  To rectify the situation after looking at it like a hog looking at a wristwatch I brought out the micrometer and figured it out. and got a weaver screw and installed it.

So what we are looking at here is that Stuff made for a weaver base will fit a Picatinny rail and stuff made for a picatinny rail system may not.  The reason I say may not is if a weaver is older  and a bit worn or not machined properly the Picatinny base may squeeze in.