Uzi barrel chamber question

[nklf]

I only said a slightly larger chamber would not speed up the ROF significantly in my opinion. However, a second barrel has other variables as well. Bore diameter, rifling depth, smoothness of the barrel can all affect bullet velocity and thus the force being applied to the bolt.

Easy enough to determine if it is the bolt, the barrel, or a combination of the two that is causing the change in ROF. That can be done by simply mixing and matching the bolts and barrels and checking the ROF in each configuration. Unfortunately once you identify the component it will be much harder to determine why. If it is the barrel, is it the chamber or some other variable. If it is the bolt is it due to difference in weight, drag on the recoil rod or something else?

 

[Slowmo]

I’m interested to see the results of this because I just got a new mini barrel where the chamber was supposed to have been hand lapped but I can still see rings in there from the machining. The rings are prominent enough to snag a loaded round, just for a moment, as it is hand fed into the chamber. So, chamber smoothness could play into this as well. Note: I have some full size Uzi barrels and all the chambers are smooth.

A sufficiently rough chamber can definitely impede extraction, which I’d expect to change ROF if sufficiently rough enough. Some designs intentionally utilized annular or helical grooves in the chamber walls for the cases to grip and resist extraction (See 409 regarding annular grooves).

Likewise, I have an M1917 Enfield with a shallow pit in the chamber that makes fire brass stick and resist extraction. The fired cases come out with a corresponding protrusion where the brass has conformed to the shape of the pit.

 

[StenAtopia]

UPDATE…..Getting closer. Changed out the barrel, installed a Spec IMI barrel. Ran both the bolts again. Noticed a significant change. Before it ran about 200rpm faster with the 1/2oz lighter bolt and now it ran about 75rpm faster. I believe the difference now is that the recoil rod hole is about .385. Very sloppy and maybe creating too much of a “pogo stick” in FA battery. I’m thinking about installing a sleeve in the recoil rod hole and reducing down to about .365. It will take up some of that slop and maybe add a tiny bit of weight to cut into that 1/2oz lighter bolt. Will report back but the barrel was at least 2/3rds the problem. Thanks all and thank you Strobro32 for your Rof timing help.

 

[Dirk Hawthorne]

Are you are using the EXACT same recoil spring each time?

Because two identical looking springs could have much different "spring rate." It changes over time, and it makes a big difference in the ROF.

 

[StenAtopia]

Are you are using the EXACT same recoil spring each time?

Because two identical looking springs could have much different "spring rate." It changes over time, and it makes a big difference in the ROF.

Only have one spring, so yes. I’d still say the 75rpm increase is either the 1/2oz lighter bolt or it’s larger recoil spring hole vs the other bolt.

 

[Slowmo]

Only have one spring, so yes. I’d still say the 75rpm increase is either the 1/2oz lighter bolt or it’s larger recoil spring hole vs the other bolt.

Just out of curiosity, do you have the ROFs for the four different combinations? I’d be interested to see how much difference that can make.

 

[StenAtopia]

Just out of curiosity, do you have the ROFs for the four different combinations? I’d be interested to see how much difference that can make.

It’s really only two combinations.

 

[Slowmo]

It’s really only two combinations.

Oh my bad. I thought you’d tested both barrels with both bolts and found a difference between the barrels as well as a difference between bolts. I must have misread your earlier post.

 

[StenAtopia]

Oh my bad. I thought you’d tested both barrels with both bolts and found a difference between the barrels as well as a difference between bolts. I must have misread your earlier post.

That way yes. Your right. Two barrels. Two bolts.

 

[Dirk Hawthorne]

Only have one spring, so yes. I’d still say the 75rpm increase is either the 1/2oz lighter bolt or it’s larger recoil spring hole vs the other bolt.

I just weighed a spare UZI FA bolt and found it to be 23.42 ounces.

If your bolt is only 1/2 oz lighter than a standard bolt, that's only about 2.2% difference in weight.

But 75 RPM difference in ROF is like 75/600 = 12.5% difference.

I would expect the difference in ROF to be more proportional to the difference in bolt weight, if the bolt weight was the culprit.

One thing you could try is to take a bolt and polish the bolt spring hole until it was like a mirror. See if the reduced friction with the spring changes the ROF.

 

[StenAtopia]

I just weighed a spare UZI FA bolt and found it to be 23.42 ounces.

If your bolt is only 1/2 oz lighter than a standard bolt, that's only about 2.2% difference in weight.

But 75 RPM difference in ROF is like 75/600 = 12.5% difference.

I would expect the difference in ROF to be more proportional to the difference in bolt weight, if the bolt weight was the culprit.

One thing you could try is to take a bolt and polish the bolt spring hole until it was like a mirror. See if the reduced friction with the spring changes the ROF.

This definitely helps me to disconcert that it’s bolt weight. Great analogy. Thank you but if I polish the bolt in question won’t that create more negative friction and maybe increase the ROF more? The faster of the two bolts, the one in question has a recoil spring opening of about .385’ish. The slower of the two bolts is .370’ish? I thought of turning down a sleeve to insert into bolt B to match bolt A to create a little friction?

 

[Dirk Hawthorne]

Yes, if the friction is lower then the gun should run faster.

I'm trying to think of all the things that could speed up or slow down an UZI:

1. The weight of the bolt.
2. The strength of the spring (AKA "rate").
3. The travel length of the spring, which can be changed by adding buffers.
4. Energy loss from friction, surfaces bearing on each other as the bolt jumps and slaps around.
5. Energy loss from throwing the oil around, if there is a lot of oil.
6. Energy loss from plowing through grease, if grease is used.
7. Hotter or weaker ammo.
8. The lubricating qualities of the oil, like Mobil 1 vs Hoppes.

Everyone already knows that 1, 2, 3 and 7 are definitely big enough effects to change ROF dramatically, and they can be tested easily.

I know that No. 6 will do the job. I tried Lubriplate grease one time and the gun would hardly run until I wiped it all off. To be fair, the ammo was weak AF.

4, 5 and 8 will definitely change ROF, the question is if it would be enough to notice.

Science establishes truth by measurement. So science is limited by whatever measuring technology exists at the time. That's why the ancient Greeks reached the highest pinnacle of mathematics, but didn't know crap about physical science. They had very crude measuring tools.

All you have is a shot timer, so if changing something is within the variation of the ammo velocity, you're not going to be able to measure it. Like if if using a crapload of oil changes the ROF by 3 RPM, you won't be able to tell. Because the spread on the normal ammo lot might be 15 RPM.

As far as the sleeve idea goes, give it a shot. Just don't get it stuck in there. Or maybe you'd want it stuck if it slowed the gun down? Personally, I kind of like when the ROF is faster, mine seems to run smoother.

 

[Dirk Hawthorne]

This thread inspired me to buy a shot timer.

 

[StenAtopia]

This thread inspired me to buy a shot timer.

lol. Well I got the steel for the sleeve. Gonna turn the outside down, insert into the spring hole and bevel it a little apply a small tack weld to keep in place. The id of the sleeve is .365, hoping that’s the wining antidote. Lol, fyi I took a better spring hole measurement and it’s .392. So that’s a lot of slop being the spring diameter is .348-.352

 

[Caverpete]

Yes, if the friction is lower then the gun should run faster.

I'm trying to think of all the things that could speed up or slow down an UZI:

1. The weight of the bolt.
2. The strength of the spring (AKA "rate").
3. The travel length of the spring, which can be changed by adding buffers.
4. Energy loss from friction, surfaces bearing on each other as the bolt jumps and slaps around.
5. Energy loss from throwing the oil around, if there is a lot of oil.
6. Energy loss from plowing through grease, if grease is used.
7. Hotter or weaker ammo.
8. The lubricating qualities of the oil, like Mobil 1 vs Hoppes.

Everyone already knows that 1, 2, 3 and 7 are definitely big enough effects to change ROF dramatically, and they can be tested easily.

I know that No. 6 will do the job. I tried Lubriplate grease one time and the gun would hardly run until I wiped it all off. To be fair, the ammo was weak AF.

4, 5 and 8 will definitely change ROF, the question is if it would be enough to notice.

Science establishes truth by measurement. So science is limited by whatever measuring technology exists at the time. That's why the ancient Greeks reached the highest pinnacle of mathematics, but didn't know crap about physical science. They had very crude measuring tools.

All you have is a shot timer, so if changing something is within the variation of the ammo velocity, you're not going to be able to measure it. Like if if using a crapload of oil changes the ROF by 3 RPM, you won't be able to tell. Because the spread on the normal ammo lot might be 15 RPM.

As far as the sleeve idea goes, give it a shot. Just don't get it stuck in there. Or maybe you'd want it stuck if it slowed the gun down? Personally, I kind of like when the ROF is faster, mine seems to run smoother.

#9 Chamber Smoothness
I’m in the process of returning a 2nd mini uzi barrel to the maker because of a rough chamber. The fired brass shows the rings in the chamber and is actually rough like a fine file. That will def change ROF even if it’s not a factor in the OP’s case.