I was just watching this video review of a DD rifle with the SLiM rail from AK Operators Union on YouTube, and at 1:25 you can see where the rail rotated slightly, maxing out the anti-rotation tabs.
https://youtu.be/QQOa5B_66oY?t=1m25s
This is one of the things I was wondering about after I installed my DDM4 rail, since the anti-rotation tabs aren't actually "fit" to the receiver and allow some play if sufficient force was applied. This is a problem if you are shooting with BUIS (which I do, occasionally). Additionally, the barrel nut is smooth with no knurling, with the rail is attached to the nut simply by a plate behind the nut that screwed into the rail... without anything providing additional friction to prevent it from rotating. Without knurling, I can definitely see how the rail could rotate on the nut.
In addition to the problems this would generate for your BUIS' zero, the tight tolerances inside the rail could also cause the rail to no longer be truly free floated, since the rail may contact the gas block. I have a BCM lo-pro gas block under my rail, and it wouldn't take much rotation for the rail to touch it.
And, if you make the ill-advised decision to bridge the gap between the receiver and the rail with your scope mount, this could cause the scope to be thrown off zero, as well.
BCM's anti-rotation solution seems superior here, in that the tabs are actually more "fitted" to the upper receiver. But I don't know how tough they are, so don't take my word for this.
Honestly, I don't think this would affect function and I'm not going to swap out my rail anytime soon, but, in retrospect, I probably should have purchased a rail that allowed me to keep my front sight post. This post is NOT a dig at DD, and I will continue to purchase DD products in the future with confidence. BUT, if you shoot with BUIS from time to time, I definitely encourage you to keep your FSP.
This post is NOT a dig at DD. If you want to attack DD, go elsewhere.
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