HK USC / UMP LOWER and STOCK Conversion

Range Report #2



First, I would like to start by saying Thank you to Sean King.  Sean asked back in January if he could be present for the second test.  After a several month delay we were finally able to have a second test and Sean was still interested in participating. 

For test 2 we were testing all the U.S. compliance parts.  The parts tested were a sear, magazine follower, and magazine base.  The sear was first tested with a 2000 cycle dry fire test and then all parts were live fired with 300 rounds of 230 grain Winchester white box ammo.  There were no malfunctions or problems while firing.  The only problem we have noted with the conversion parts is accelerated wear on the HK hammer.  This wear does not make the part unsafe but does limit it's life.  Because of this, we are suggesting this conversion only for the causal shooter.  We will begin developing another compliance part to remove the wear issue.  Our conversion sear is not used in our conversions for registered Short Barreled Rifles.  Read on below for more testing details.




Click above button for Test #1 results page


Test #2 : Complete

This test was to test all parts for this conversion.  New parts tested were a stainless sear and an aluminum magazine follower. 




Testing details

The first part of our test was to test the new sear for extended use.  All parts were assembled into a USC lower.  The test was preformed by pulling the trigger to release the hammer letting it fall onto a hammer stop, recocking the hammer, then releasing the trigger to reset the hammer.  This was done for a total of 2000 times. 

Hammer and sear operation was checked periodically for positive engagement.  This is done by pulling the trigger slowly and observing the hammer.  With positive engagement, before the sear releases the hammer, the hammer should start to move rearward to a more cocked position.  If the trigger is not pulled far enough to release the sear from the hammer, when the trigger is released, the hammer should move back to it's original position.  Neutral engagement is when the hammer does not move reward as the trigger is started to be pulled or the hammer does not move back to its relaxed position when the trigger is released before the sear releases the hammer.  Negative engagement is when a slight pull of the trigger will allow the sear to slip away from the hammer or when the hammer can be pushed on and it will fall without any pull of the trigger.  Both neutral and negative engagement is considered unsafe.

Engagement was checked every 100 cycles for the first 1000 cycles of the test.  During the last 1000 cycles, engagement was checked every 50 cycles.  At no time did the hammer and sear show anything but positive engagement.  By the end of the test the distance the trigger had to be pulled for the sear to release the hammer had been reduced.  At no time during testing was the hammer and sear deemed unsafe.

After dry cycles were complete, the sear just tested and a new hammer were installed into a converted UMP lower assembly and tested with a fully converted USC.  During the 300 round test, no problem of any kind was experienced.


Hammer wear

To the left is a photo of 3 hammers.  The one to the left is a new hammer. 

The hammer in the middle is from the 300 round live fire test.  You can already begin to see wear on the sear notch.

The hammer on the right is from the 2000 dry cycle test.  You can see just how worn the sear notch has become.  Although this hammer never showed any unsafe action during testing, the amount of wear will result in a shorter useful life when using our sear as compared to using an HK sear.




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