Modifying a Top Notch Stock

For use with the USC to UMP conversion


After examining a Top Notch (TN) made UMP style stock, we found it is not a direct replacement for an HK manufactured UMP stock.  The differences with the TN stock and an HK stock are as follows:

    1.  The TN stock's hinge pin hole is larger then the HK's hole.

    2.  The location of the hinge on the TN stock is in a slightly different position then the HK stock.

    3.  The TN stock's latch button has a shorter engagement surface then the HK's and it is shaped slightly different from the HK.

The first photo shows an unmodified TN stock placed on our conversion hinge / latch assembly.  The TN stock can not sit flush against our assembly which can be seen by the gap between the back of the assembly and the front of the stock.

Unmodified TN stock

Unmodified, the TN stock's latch button will not engage the latch opening of our assembly and the hinge pin holes do not align.  We also found during the examination of the TN stock that its latch button has a more looser fit where it moves in the stock then the HK button does in the HK stock.  The TN stock latch button spring is also weaker then the spring in the HK stock.

To make the TN stock work with our conversion we do the following:

The front of the hinge on the TN stock has material removed to allow the stock to seat properly in the Hinge / latch assembly.  The hinge pin hole of the stock is filled with fiberglass then re-drilled to match the holes in the  Hinge / latch assembly.  Small plastic shims are glued into the channels of the TN stock's latch button to remove any excess movement.  The engagement surface of the latch button is then modified to properly fit the latch opening of the Hinge / latch assembly. 

The photos below show the fit of the TN stock after modifications are complete. 

Because the TN stock's Latch button is shorter then the HK's, when fully extended, it does not engage the latch opening as much as an HK latch button does.  This does not appear to be a problem.  After modification and fitting the stock locks solidly open.

Modified TN stock


Modified TN stock Latch Button


Modified TN stock Hinge



TN stock Latch Button does not

extend as far as an HK unit



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