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It's entirely possible that the WE delta ring is off-spec.
Try getting a real-steel delta-ring.

i could be wrong, but doesn't this rail takes a round end cap?
nevermind, i was wrong, just get a real steel end cap, should be able to get some for cheap price

Those things are kind of tricky to persuade in. Can you pull off the cap and delta ring and see if the fit on their own?

I had a KAC M5 rail and M16a3, they do fit. PITA to install though.

Is that lower rail on backwards?

A real KAC rail WILL fit a WE ring system.

You will be cussing and swearing. You will bleed. It will never come off. But by god will it be solid.