Two things occur to me.
In addition to the breechblock fitting in the receiver rails, the receiver must also fit in the frame. If the receiver extension is spread too far, it may bind on the frame.
In operation, the breechblock does not go clear to the end of the receiver extension--its tavel ends about 3/4 inch before. The bitter ends of the rails may be close enough so that the breechblock is tight to reassemble, but in its operational travel it may be just fine.
--Dwight
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