I bought inexpensive PEPR mounts, and found out they were a poor substitute for the real deal. I put 2 of these mounts on my rail. The 2nd was rotated 180 degrees so the mounts were facing each other. Where they should have been perfectly aligned, there was a gap of 0.070". So the single mount was off by 0.035" over a 3" span. If my nearly forgotten highschool trig is correct, it's about 0.7 degrees off. Here's a picture of the 2 mounts facing.