Dumb question:  What is the purpose of the very small hole on the top portion of the mag fence?  Thanks!

[b] The only dumb question is the one not asked. That said , I'm not sure I understand what your referring too. Are you talking about the magazine itself ? Do you mean that little notch on the side?  Try too explain in more detail .

Okay -- here goes!  It is a very small hole on the top portion of the fence...about 1 inch directly behind the front push pin (and parallel to it).  

I don't normally see a hole like this which is why I'm asking.

It's for putting in a drop of oil every couple of years or, allowing any water that's gotten in there (think Viet Nam jungles) to seep out.

The first ARs/M16s didn't have the hole, and a 1966 GI Tech manual I have has instructions for GI armorers on how to set up the lower to drill the hole.  Apoparently they had discovered by that time that moisture could get in, and rust the spring/plunger into place.

Oddly enough, I have a Rock River lower w/o the hole. Really doesn't effect anything one way or another in a civvie piece, but I'll be sure to drill one if they ever send me to the jungle with it. [;D][:D]

[b]It's a pinhole camera placed there by ATF!![/b]

If you will look in the hole you should see a shiny spring. This is your pivot pin spring which puts pressure on the pivot pin detent which keeps your pivot pin from falling out. I put a toothpick (or whatever) in the hole to hold the spring back while installing the pivot pin.    

LMAO

Are you talking about the tiny hole that you can see the pivot pin detent spring through? If so, I dont know why its there.