Peep Sight on, but...

[corelokt308win]

I got a peep sight from NEF, and put it on.  The rear mounting screw seemed to barely even grap any threads, maybe one full turn.  Anyone else notice this when mounting one of these?  The front screw was fine.
  Also, I've been reading posts here, and seen where sometimes the rear peep sight is shimmed to bring it to height, and where sometimes a front sight off a Hornet barrel is used because it is the tallest sight available.  Why would the back sight need shimming to make it higher and a higher front sight also be needed?  Couldn't one or the other be left lower?
  Anyone know how to measure to figure the height of sights needed?  Would I measure the diameter of the barrel and add the height of the rear sight when the peep is in the middle of its travel, then do the same to the front of the barrel and the amount needed to make the front sight the same as the back be right?

[raynor]

I have a Sidekick with the Williams sights, no shimming was needed for either sight and I am using the stock front tru-glow.  Plenty of adjustment left for up and down on the Williams peep.  Shooting Nosler sabots with 240 grain bullets and 80 of loose 777. Maybe with heavier bullets you'd have to shim to get the correct POI.

Check to see if there's a burr inside the screw hole on the Williams peep. I've seen that on a few, it won't let the screw go down in far enough.