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

[amphibian]

I have mentioned doing this before but never took the time to document it and I have some interesting results on my website here: https://c3junkie.com/?page_id=2028
For those too lazy to click on my website, I will copy some of it here:
All ammo used was my 3.7Gr Titegroup under a 147Gr projectile. All testing done yesterday.
Baseline.

The test below has expected results:

Test below has expected results and I think this what I did a long time ago before the first run of BarrelXchange Mini Heavy bolts came out. Again, the bolt is lighter than a FS UZI bolt so obviously it should run faster than the full size bolt.

This test is what has me confused, I would have logically thought being 1.4oz HEAVIER than the FS Uzi bolt that we should be running less than 577 but it is faster.
I know the timer is good and you can definitely hear and feel the difference in the cyclic rates. I presume that the location of the mass is what is different here. Very interesting nonetheless.

 

[mattnh]

That is an odd result.
Perhaps the spring is insufficient and bottoming out or something like that

 

[amphibian]

Uh...you are right...that is it. The spring is under more compressive force using the Mini bolt. So you would think should be able to get the FS at 700 w/o spacers!!

ETA: I just checked and sure enough the spring is going solid height doing this. So bolt travel ends up being about 3" doing this. Good thing I never ran this long term as having all that load concentrated on the guide rod is probably not a good idea vs spreading across the surface area on the back of the bolt.

Now I want to mess with springs to see what can be done. I tried reaching out to vendors some time ago to get a flat spring that would fit inside the UZI bolt but never got anywhere with that.

 

[mattnh]

Imho, very interesting experients

 

[amphibian]

So thinking about how little the red dot moved with the Mini bolt, I want to speed up the full size but I don't want to cut coils on the spring as cutting coils also makes the gun run slower and can also decrease reliability. Lightening the bolt is the next option.
I used to have a full size bolt where someone milled the slot to the left side of the recoil spring so I did that.

I milled the bolt like the one I had but that is only 1.4oz reduction.

Haven't shot it yet but will be interesting to see how it does.
Was also thinking that some people may already know how this plays out as I thought BarrelXchange sold some heavy .45 bolts that could be run with all the weights removed like I had done with the Mini Heavy bolt?

 

[124NATO]

Hey Amphibian

Just an idea if you could make an extension sleeve that fits the mini bolt you could put the spring tension back to where it normally sits for the full size. You would probably have to make a custom guide rod that will fit past the reduced bolt hole and then make the flair to pass through the custom extension.
Hope that makes sense?

 

[amphibian]

Hey Amphibian

Just an idea if you could make an extension sleeve that fits the mini bolt you could put the spring tension back to where it normally sits for the full size. You would probably have to make a custom guide rod that will fit past the reduced bolt hole and then make the flair to pass through the custom extension.
Hope that makes sense?

I briefly thought about doing an extension but don't want to make any permanent changes to an expensive Mini bolt. I would think I would need to weld on it and don't want to do that.
I was thinking doing so would also involve boring out the reduced hole and moving forward to replicate the same full size spring captured length.

You are saying to leave the reduced bolt hole the same and I can't visualize how that wouldn't have the same issue I'm dealing with.....so figured milling on the FS bolt would be easier and those bolts are still available today...definitely not as cheap as they were years ago though.

 

[TSPC]

The recoil springs used on full size registered bolts have less coils and will allow the mini bolt to go all the way back in a full size Uzi without going solid.

 

[amphibian]

The recoil springs used on full size registered bolts have less coils and will allow the mini bolt to go all the way back in a full size Uzi without going solid.

View attachment 39807

Interesting...I have/had some of those recoil spring assemblies....need to see if I can dig through my stash for one. I don't understand why they would be that way for registered bolts though? I thought most registered bolts were standard SMG bolts that were milled to clear the bolt blocking rail. I know the semi closed bolts are not counterbored as deep as an SMG bolts so if the registered bolt was made from a Semi bolt that would make sense. I'm guessing there are some out there like that but wouldn't think it is the majority?

 

[TSPC]

Interesting...I have/had some of those recoil spring assemblies....need to see if I can dig through my stash for one. I don't understand why they would be that way for registered bolts though? I thought most registered bolts were standard SMG bolts that were milled to clear the bolt blocking rail. I know the semi closed bolts are not counterbored as deep as an SMG bolts so if the registered bolt was made from a Semi bolt that would make sense. I'm guessing there are some out there like that but wouldn't think it is the majority?

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.

 

[brenbuilds]

Out of curiosity, have you chronographed your 9mm load? I wonder what the difference in cyclic rate would be suppressed and un-suppressed with these different setups.

If you have some spare recoil springs you can reduce the wire diameter by carefully sanding the outside diameter of the spring on a belt sander; it works well in applications where you don't want to change the springs O.A.L. Cool test. Thanks for doing this.

 

[BobMG]

I made this contraption several years ago to speed up the full size. I never liked limiting travel with the hard buffer block so I cut one down slightly in length, drilled it out to fit a much heavier spring for a cushioned effect. The spring is held in with safety wire from the back. Didn’t measure the rate, but it’s quite a bit faster.

 

[amphibian]

Out of curiosity, have you chronographed your 9mm load? I wonder what the difference in cyclic rate would be suppressed and un-suppressed with these different setups.

Yes, looking at some old data I have, my load had an average of 947 fps.
I didn't want to do suppressed testing right now as there are so many different cans w/ varying levels of backpressure and wanted to minimize the info right now. I have posted suppressed FA cyclic rates many times but I am trying to get organized and actually put this stuff on my website so I don't forget where I put that info.

If you have some spare recoil springs you can reduce the wire diameter by carefully sanding the outside diameter of the spring on a belt sander; it works well in applications where you don't want to change the springs O.A.L. Cool test. Thanks for doing this.

Yeah...would rather not do that.

 

[amphibian]

I made this contraption several years ago to speed up the full size. I never liked limiting travel with the hard buffer block so I cut one down slightly in length, drilled it out to fit a much heavier spring for a cushioned effect. The spring is held in with safety wire from the back. Didn’t measure the rate, but it’s quite a bit faster.

Thanks, may give that a try too.

 

[cvasqu03]

Sorry, slightly off topic, but how were you able to mill on the FS bolt without heat treating? I'd heard those bolts needed to be softened up somewhat in order to be worked on. What kind of bits did you use?

 

[chili17]

They have cut fine for me with carbide

 

[trilogymac]

Like Chris said carbide works fine. I have done it numerous times as well.

 

[amphibian]

Yes, I used carbide for this.

 

[b_saan]

Seems like an awful lot of engineering work to make something to do what the BlackJack Buffers already perfected.

 

[amphibian]

Seems like an awful lot of engineering work to make something to do what the BlackJack Buffers already perfected.

Not really, I've tried buffers in the past and you are not reducing reciprocating mass doing that. Also I think it beats the gun up like the late BWE also said in this video:

As I mentioned previously, you can clearly see and feel less movement with less mass. In the video I link above, Richard at BWE also mentioned to be careful making the bolt too light but I would think I am fine as I am still 1oz heavier than a factory IMI Mini bolt.
The travel is the same.