Differences Between Old Style and New Style

UMP Lower Receivers

HK has made several changes to the UMP lower receiver over the years.  For our conversions we will group receivers into two different types.  We will call them "Old style" and "New style". 

The major differences between the Old style and New style receivers are a rib in the area of the auto sear, the wall in the rear of the lowers, the location of the auto sear pin hole, and the material the receivers are made from.

The Old style receiver lacks a rib which the New style receiver has.  This can be seen in the photos below.

The Auto sear pin hole sits much lower in the Old style receiver.  This can be seen in the last photo.

The wall in the rear of the old lower will not allow installation of USC fire control parts unless the wall is modified.

The plastic the Old style receiver is made from is not as durable as the New style receivers.  Because of this, Old style receivers are more prone to scratches and damage.  The plastic also appears to melt at a lower temperature making it more difficult to drill cleanly.

From what we have observed, Old style receivers are more likely to be purchased with demilled UMP parts kits.  Newly ordered receivers have all been of the New style.

Because the location of the auto sear hole does not allow the Old style receiver to fit our drilling jig and because of the difficulty we have had drilling these receivers cleanly, we will not be accepting Old style receivers for conversion work.

Old style lower

New style lower

Old style lower

Spacing on receiver Auto Sear holes

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