Re: MilStd 1913 rails
Anytime you put a tool to a piece of material you run the risk of it distorting. It's a natural by product of machining. You relieve stress inside a chunk of steel by cutting on it.
Another way of course is to have the pieces normalized prior to working on them. This is a process where the parent material is heated/cooled at a specific rate to allow things to settle down a bit.
Basically it's like this: If you are making a 1913 rail expect the piece to "sway it's back" when you cut the little slots. It'll tend to want to bow up on the ends and rock like a chair. Just part of the deal. If you were to flip it over and cut the bottom the same way it'd probably bend back to almost straight again.
Not a big deal, just stick em in a press and bend them back straight.
Alot of aluminum rails are made as an extrusion meaning they have a die made and then "poop them out" in the general shape. From there you just zip the slots, drill some holes and maybe contour the corners and your done. Because it's an extrusion the grain of the material is a little different and they behave a little differently.