Take the rear sight apart and file the bottom of it a little bit .
Pics and info can be found at
2. FITTING THE NM REAR SIGHT APERTURE
http://www.fulton-armory.com/\faqs\M14-FAQs\NMRearSight.htm
Copied this from Hungry
How to install your rear sights (after you took them off)
Some of you might have upgraded your rear iron sights with some USGI M1 Garand or USGI M14 replacements. Here's how to do it, just OFF the top of my head. I do this so frequently, it's like shaving my face!
GREASE the threads of the elevation pinion screw RFN (right now) and then grease the threads of the windage knob as well. This helps prevent cross threading especially if you are using NM threaded/pitch knobs/rear sight base.
Just for your reference, you experienced Garand sight users: The USGI M1 Garand thread pitch is 16 tpi (threads per inch). The NM spec to obtain 1/2 MOA per klik is 32 tpi and hence is easier/prone to cross threading. Been there, done that! Had to help another CGNutter recover from this error. years ago!
I start with the elevation knob pushed in (left or portside) as far as it can go.
Then I center the elevation pinion screw in the center of the windage knob 'window' and attempt to 'screw' windage knob into place.
Keep up the pressure (windage knob from the right or starboard side) and screw the windage knob in place until it's flush. This requires a 'feel' that cannot be described on the internet.
Sometimes I will wiggle the rear sight base left and right until the windage knobs settles as tight as it can go into the rear sight base.
Once you are flush and even and all the knobs have no further to screw in, you will know that the knobs are 'home' position because the windage knob now 'clicks' and the rear base moves left and/or right per click.
I now take a flat blade screwdriver and tighten the windage knob pinion screw 1 or TWO half clicks to obtain the tension needed to move the rear base left and / or right without having to push on the elevation pinion screw head from the left to the right with my left thumb. Go and do that right now with your M14 or M1 Garand sights. As you move the windage left or right, now PUSH on the slotted pinion screw(elevation knob) towards the windage knob. Notice the windage knob is EASIER to klik (left or right) ?
I'm hoping this will work for you. Works for me all the time!
mates!
Barney