Full auto Full size UZI cyclic rate testing w/ Mini bolt in the full size

[cvasqu03]

So, are you going to try to lighten it that extra ounce or are you pretty much satisfied with where it is?

 

[amphibian]

So, are you going to try to lighten it that extra ounce or are you pretty much satisfied with where it is?

I need to test then take it from there.....I hope I didn't take too much off already...

 

[amphibian]

I’ve wondered that as well. Only thing I can think of is to compensate for the weight loss from milling the bolt. However, I have a registered bolt and it runs fine with a standard recoil spring.

Reuben has four registered bolt Uzi's for sale right now and two of them have this type of spring so they’re somewhat common on registered bolts.

I couldn't find my spring like that in my stash. I reached out to my buddy that I sold one of my FS UZI's to a long time ago and he has it so I borrowed it for future testing.

I also managed to test the lightened bolt yesterday. Only 60RPM faster:

 

[Chef]

60 RPM is a nice increase.

 

[00wingzero1980]

So I can replace my full size uzi bolt for a min-uzi bolt to increase the ROF?
Any other modifications needed?
Any increase in wear

Thanks

I couldn't find my spring like that in my stash. I reached out to my buddy that I sold one of my FS UZI's to a long time ago and he has it so I borrowed it for future testing.

I also managed to test the lightened bolt yesterday. Only 60RPM faster:

 

[amphibian]

So I can replace my full size uzi bolt for a min-uzi bolt to increase the ROF?
Any other modifications needed?
Any increase in wear

Thanks

My testing isn't done yet....if you use the typical UZI SMG recoil spring assembly, the spring goes solid height using a Mini bolt in a full size which then concentrates all of the force into the guide rod vs spreading it across the back of the bolt. I don't think that is a good idea and may damage the receiver if done long term.

User @TSPC mentioned using another spring that has more distance between the coils that doesn't go solid height. I just borrowed one from my friend that I sold one of my FS UZI's to like 15 or so years ago. I haven't tested that spring yet. I hope to test it this weekend with an unmodified FS bolt, my lightened FS bolt, standard Mini bolt, heavy Mini bolt and heavy Mini bolt with the weights removed.

There is also the issue that the spring @TSPC mentioned I guess were around 20+ years ago and I suspect they are not IMI in origin so I don't know of an equivalent source to even replicate the results. I won't try to replicate it if the results aren't desirable either so that is why I need to do some testing.

Also note that even if it does work putting a Mini in a full size, you will need a custom cocking lug made like you would use to run a Micro bolt in a Mini. There are threads on that here. I have only one of those left and will TIG weld some more up at some point.

 

[trilogymac]

Robert,
Good morning.
Is there a possibility that the bolt face or barrel can be damaged if dry fired? The front of a mini bolt wont contact the trunnion of a fullsize because of the shorter length?
Bill

 

[amphibian]

Robert,
Good morning.
Is there a possibility that the bolt face or barrel can be damaged if dry fired? The front of a mini bolt wont contact the trunnion of a fullsize because of the shorter length?
Bill

Yes, that is possible too. I'm only doing this out of curiosity. I think the best way is just run a lightened bolt to avoid all that. Less reciprocating mass with the lightened FS bolt. However, comparing the stock configuration to my lightened bolt they are so very close that I don't think anyone would even notice a difference unless fired back to back and paying attention.

Since I already milled on that FS bolt, I may mill on it some more to get another 1 once or so out of it to closer to the Mini bolt weight or maybe a little under and then close out this experiment.

I would like to get back to the 700RPM rate but if it requires reducing the travel, I don't want to do it. As I mentioned before, I tried sourcing a flat spring that would fit in an UZI bolt and gave up.

I know a lot of this seems to indicate that the travel distance is all there is to consider but springs do play a huge part. If you compare a Mini UZI in open vs closed bolt, the travel is pretty close but the Mini using the additional striker spring really jacks up the RoF. Open bolt in the 900's and then closed bolt around 1500 - 1700.
However, in the full size to add a spring like the other guy did, you need to have something to hold that spring which then reduces travel.
I was also thinking about making a spring loaded piston to put in the cavity I milled out on the right side to boost the cyclic rate as well but alas, there are plenty of other projects I could be doing.

 

[trilogymac]

I have a full size closed SMG bolt and the cycle rate is considerably higher. I attribute most of that to the striker spring and shortened stroke.

What I found is that when the cycle rate is moderately increased in the full size with an open bolt, it "feels" much smoother, but when shot on paper, the full auto groups didn't confirm it. My groups with 2-3 round bursts were actually tighter with the slower rate of fire which surprised me. Perhaps it is just the way I shoot. I know its a bit off topic but it relates to cycle rate so I figured I add that.

 

[chili17]

Ill dig around. I milled out alot of weight off a full size bolt. It was pretty fast from what i remember. Tossed it in a parts box years ago,, I'm sure its still there

 

[amphibian]

I have a full size closed SMG bolt and the cycle rate is considerably higher. I attribute most of that to the striker spring and shortened stroke.

What I found is that when the cycle rate is moderately increased in the full size with an open bolt, it "feels" much smoother, but when shot on paper, the full auto groups didn't confirm it. My groups with 2-3 round bursts were actually tighter with the slower rate of fire which surprised me. Perhaps it is just the way I shoot. I know its a bit off topic but it relates to cycle rate so I figured I add that.

That is great info!! I need to tell my buddy that so he can try that out.

 

[amphibian]

I did some more testing over the weekend and it appears to me that the difference has little to do with the decreased travel but because the spring is under more load when you use a Mini bolt in a full size.

As you can see, the results are almost the same using the 'registered bolt' spring which allows full travel vs the IMI spring.

So.... based on these results, I plan on making a bushing to insert in the recoil spring hole of the full size UZI that will replicate using the shorter Mini bolt in the full size.