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Ok, I figured it out.
On the front of the handguards, there are two channels which lock into the end cap. On the MI rail, when the two handguards are together, that gap is about 3/8 of an inch. On the stock hanguards, the gap is a 1/4 inch.
To make the MI rail fit, you need to filedown the front of the MI guard at that gap so that it fits completely into the tabs in the cap. Otherwise, it doesn't seat far enough to mate the sprocket with the groove at the back of the handguard. Boy do I feel stupid.
Top is on, filing down the bottom now.
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