Uzi barrel chamber question

[StenAtopia]

So if a barrel has excessive or an oversized chamber would that cause an increase in Rate of Fire?

 

[NorincoKid]

I would imagine the opposite (a sticky, tight chamber) could theoretically slow down ROF since it takes more time/effort to extract the casing so.......perhaps yes? A looser chamber could/would/might/should cycle faster in comparison to that?

But I'm not sure it would even be noticeable?

It's testable with some 9mm barrel blanks and a reamer...

 

[slimshady]

I would think no noticeable difference. Too loose you get split cases and possibly a jam or a blown case. Too deep and you get no bang.

 

[arch stanton]

So if a barrel has excessive or an oversized chamber would that cause an increase in Rate of Fire?

No not at all

 

[StenAtopia]

So I am running a mini Uzi. First time I ran it it seemed normal( for a mini) . Second time I ran it it seemed to run a quite bit faster. Same ammo but different barrels and bolt. I have two mini bolts. I’m trying to figure out why it ran faster. I weighed each bolt. One of the two, the one I ran today weighs .45 of an ounce lighter. So one is like 19.6 oz the one today is 19.1oz-19.2oz. Only thing I thought was the barrels. The barrel I ran today the chamber is much bigger. Again same ammo. FYI. Open bolt

 

[slimshady]

So swap bbls back but not the bolt and see if makes a difference

 

[StenAtopia]

So swap bbls back but not the bolt and see if makes a difference

Yeah gonna try that next session. Just weird unless 1/2oz makes that much a difference in ROF? Thanks

 

[Slowmo]

Is it consistently faster or more like quick little stutters of faster fire? I experienced the latter with some reloaded rounds where the caseheads were oversized and not plunk testing smoothly.

 

[StenAtopia]

Is it consistently faster or more like quick little stutters of faster fire? I experienced the latter with some reloaded rounds where the caseheads were oversized and not plunk testing smoothly.

Faster in general. It’s not an ammo issue, I’ve been using the same ammo lot the past few months. Probably barrel? Or maybe the bolt being 1/2oz lighter than the original? Confusing, well I have to do a range session I guess and drill down to what’s going on.

 

[Dirk Hawthorne]

Just taking a guess, I would say that a smooth but oversize chamber could speed up ROF. It's possible, anyway.

If you run weak ammo or hot ammo, you can see a noticeable difference in ROF, correct?

That's because during the time the bullet is in the bore, the case is giving the bolt a smaller or larger thump. Because the force on the case is different.

And so the initial rearward velocity of the bolt is lower or higher.

SO, if the grip of the case on the chamber is a significant factor on slowing down the case, a big chamber could change the ROF. Because there is a limit to how much brass flows under pressure.

Please report back with your results, it's an interesting topic.

 

[nklf]

I would think the half ounce lighter bolt would have more impact on ROF than the oversized chamber, but I have been proven wrong before.

 

[arch stanton]

I would think the half ounce lighter bolt would have more impact on ROF than the oversized chamber, but I have been proven wrong before.

Yes this exactly an oversized chamber has nothing to do with an increase in the rate of fire. What an oversized chamber can do is cause loss in accuracy and more pressure.

 

[Slowmo]

It’s not exactly analogous, but amphibian experimented with a fluted chamber (HK barrel) in his CMMG to see if it helped the extraction issues, and the combination actually had lower ROF than the unfluted barrel. Of course that platform is delayed and involves a rotational component, but I’d expect flutes to be similar to an oversized chamber in minimizing case-chamber friction.

 

[StenAtopia]

I would think the half ounce lighter bolt would have more impact on ROF than the oversized chamber, but I have been proven wrong befo

So a 1/2oz lighter bolt will cause 150-200 rpm increase. Guess we will see when I try again on Monday. But I guess it’s plausible?

 

[slimshady]

Also bolt fit and finish as well as lube would vary friction and thus bolt speed.

 

[David Hineline]

You have the two bolts and two barrels in question, there are 6 possible combinations to test, I think the man with the variables could easily do 6 tests and come up with an answer to his own question.

 

[slimshady]

It's late and I'm half asleep but it seems to me you install bbl A and test with bolt 1 and then bolt 2. Then swap in bbl B with bolt 2 then bolt 1. That's 4 by my count, what are the other two combinations?

 

[StenAtopia]

You have the two bolts and two barrels in question, there are 6 possible combinations to test, I think the man with the variables could easily do 6 tests and come up with an answer to his own question.

Of course man, but I can’t shoot everyday. If you read above I plan this Monday. Was just stumped at the “initial” run. But yea trial and errors were the future endeavors. Thanks.

 

[Dirk Hawthorne]

So a 1/2oz lighter bolt will cause 150-200 rpm increase. Guess we will see when I try again on Monday. But I guess it’s plausible?

The lighter something is (in ounces), the lower its mass is.

And mass is defined as the resistance to acceleration.

A 9mm cartridge is going to exert an average force of somewhere in the neighborhood of 3,500 lbs on the bolt face for a split second.

F=m x a

So all other things being equal, the acceleration of the bolt will be greater if the bolt is lighter. And so the initial velocity of the bolt will be greater.

Of course, things don't always behave the way you expect them to, because there is sometimes a lot going on that you might not know about. The outcome of experiments are sometimes way different that what is expected.

In this case, though, it's just a lump of metal attached to a spring, slapping around in a metal tube. I would amazed if a lighter bolt didn't speed up the ROF.

 

[Dirk Hawthorne]

It’s not exactly analogous, but amphibian experimented with a fluted chamber (HK barrel) in his CMMG to see if it helped the extraction issues, and the combination actually had lower ROF than the unfluted barrel. Of course that platform is delayed and involves a rotational component, but I’d expect flutes to be similar to an oversized chamber in minimizing case-chamber friction.

It would be really interesting to see what the actual drag force is from the cartridge grabbing the chamber wall.

To see if it is significant compared to the thrust from the chamber pressure.

I'm sure somebody clever has found a way to test that, but I couldn't find anything on it.