[dohcacr]

To add: The above measurement was taken using a mic. I have an instructions sheet for installing a muzzle brake that refers to it as 1/8" roll pin.

+1 At US hardware stores you should be looking for a 1/8" roll pin.

 

[dohcacr]

many, many thanks for the info.
Well when I got the roll pin through, the sight would cant back and forth. .

If you are referring to the sight "canting back and forth" along the long axis of the barrel, that's pretty normal for the slip-on type of sight. In order to defeat that teeter tauter motion, you would need a tighter fit (pressure fit) which would cause problems for some novice installers. You can drill and tap a hole for a set screw, on the bottom side of the sight, or do as I did, and add a brass shim to the back/bottom side of the sight.