Full Size UZI cyclic rate testing

Baseline configuration:

I was never a fan of adding spacers to the FS Uzi’s to increase the cyclic rate.  In the past, I have put Mini bolts in the full size but never took the time to really document the results and only did it in passing and never doing this long term.  While the travel distance is the same the spring will go solid height doing this and results in a bolt travel of only 3 inches.  I suspect finding an alternative spring will change all all these results.

Below are the results putting the stock IMI Mini bolt in the full size, with zero changes other than swapping the bolt.

Below is another data point I figured I would try since I still have this factory IMI bolt that a friend modified for me back before BarrelXchange came out with his heavy bolt.  Again, zero changes other than swapping the bolt.

Lastly, is putting the BarrelXchange Mini Heavy bolt in the full size.  Again, zero changes other than swapping the bolts.   This bolt is actually 1.4oz heavier than the stock FS UZI bolt and you would think I should be getting slower than the 577RPM I got with the factory IMI bolt but is actually faster even though we just went to a heavier bolt.  Again, bolt travel distance is the same as you can see from the second picture on this page.  The reason is that since the bolt is shorter, the shoulder that spring interfaces with on the bolt, it reduces the bolt travel.  I measured 3″ of bolt travel and then the spring goes solid height so not a good idea to run this long term as you are forcing all that load on the guide rod on the receiver instead of the surface area of the back of the bolt.

I really liked how the dot would barely move with the higher cyclic rate but running with a spring going solid height isn’t good.  I had an UZI bolt that I got somewhere a long time ago that had a slot cut in it to lighten it but I gave it away.  I decided to modify one of my IMI bolts to do something similar.

Baseline test of using the ‘registered bolt’ spring with a stock setup is only 25 RPM faster than an IMI spring as tested above.

Tested the ‘registered bolt’ spring with my lighted bolt is now 65 RPM faster than stock (577 RPM)

‘Registered bolt’ spring with custom 21.3 oz Elkonite Mini bolt only 9RPM faster than previous test with IMI spring.

‘Registered bolt’ spring with BarrelXchange Heavy Mini bolt yields almost the same results as above.  These tests seem to indicate the increase in cyclic rate have little to with the reduced travel but because the spring under more load in the Mini bolt vs the FS bolt.