I am working on installing a Knight Armament RAS rail system on my S&W M&P 15 carbine. I can get the front part of the upper rail system in, but the rear won't go down all the way. It seems like the little forklike clamp can't get past the barrel nut, which it is supposed to go under.

What can I do?

Never mind, I drove out the spring pin holding the rear clamp to the upper rail and removed the clamp. The rail fits on fine without the clamp piece and doesn't wiggle at all.