I recently installed a Bob Jones lens on my training rifle and noticed I was getting shading at the bottom of the aperture.  It turned out to be caused by a very slight backward tilt of the rear aperture.  This tilt can become very noticeable when you insert the ¼” length of rubber tubing that retains the Bob Jones lens.  The tube creates a tunnel and any misalignment will cause the shading when you look through the rear aperture.  The fix is relatively simple.

1.  Place the upper in a cradle or vise and level it.  I placed my level on the end of the barrel.
2.  Using a small level, check the level of the rear aperture hood, front to back.
3.  If tilted backwards, use a set of feeler gauges under the rear sight aperture to determine what thickness shim will level the aperture.
4.  Purchase a second feeler gauge for shimming material.  Cut a piece to size ensuring the shim material is flat after cutting it.  Also, depending on the bonding agent you use, you may need to reduce the shim .001”.  I used Loctite super glue to hold it in place.

Leveling my rear sights has really brightened up my sight picture.  My training rifle, an A3 upper with RRA NM carry handle needed .004”; my match rifle, an RRA A2 NM upper with a Micro-sight needed .011”.

I have thought about tapping the unused aperture and inserting a leveling screw.  A ¼-32 set screw would be idea but may be hard to come by.  A ¼-28 is easier to find but it will need to be relatively short to clear the hooded aperture.

Hope this information is helpful.

Art

I tried the Bob Jones and noticed the same problem.  I know they make a little tool for bending the hooded sight, but I never bought that.  I finally went to a pair of Knobloch shooting glasses with a +.75 lense for the right eye.  For me it worked out better than trying to install a Bob Jones lense in multiple different guns.