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Well... you *could* have someone machine the outside of the pin down, but that sure is going about it the hard way. It's much easier to just machine the plastic lower. I have a fixture that holds it at the exact angle, and feed a (IIRC) 11/32" end mill into the notches of the hooks, to the center plus about .020". It lets the lower drop on but requires a tiny bit of pressure at the rear to install the stock block so the lower doesn't have any clearance to rattle. Also, the plastic hooks are not solid plastic - there is a steel frame inside the UMP lower that extends into the hooks. When they are machined to fit the USC receiver pins, they are exposed by the end mill.
 

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Technically anything is removable............. but yes, that is the more difficult way of going about it. I'd rather mess up a lower I ccan get for a couple hundred dollars, than mess up the reciever that will cause me to have to buy another 2k gun.........
 
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