Uzi barrel chamber question

[StenAtopia]

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.

I get it and understand. I can see “ounces” making a difference but a 1/2 ounce making it fire 150-200rpm faster is a hard sell. I’m still banking on the barrel that was changed for the increase in rof. Guess I’ll find out Monday or Tuesday. Very interesting indeed for sure.

 

[Dirk Hawthorne]

I get it and understand. I can see “ounces” making a difference but a 1/2 ounce making it fire 150-200rpm faster is a hard sell. I’m still banking on the barrel that was changed for the increase in rof. Guess I’ll find out Monday or Tuesday. Very interesting indeed for sure.

I misunderstood your comment, I thought you meant the idea in general.

I think you're right. Taking a guess, 1/2 oz is probably good for 20rpm to maybe 50rpm change in ROF. 150 sounds way high.

 

[StenAtopia]

I misunderstood your comment, I thought you meant the idea in general.

I think you're right. Taking a guess, 1/2 oz is probably good for 20rpm to maybe 50rpm change in ROF. 150 sounds way high.

Thanks for your reply. You have and I mean this truly, very insightful!!

 

[Slowmo]

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.

Chinn’s book series on MG design might have something. I’ve always wanted to read through it, just never have found the time.

https://www.ibiblio.org/hyperwar/USN/ref/MG/index.html

 

[Dirk Hawthorne]

Thanks for your reply. You have and I mean this truly, very insightful!!

Thank you for saying that, but in all modesty this topic is directly related to my job.

An UZI is literally this:

 

[Dirk Hawthorne]

Chinn’s book series on MG design might have something. I’ve always wanted to read through it, just never have found the time.

https://www.ibiblio.org/hyperwar/USN/ref/MG/index.html

wow, that is a lot of information.

Thanks for sharing that.

 

[arch stanton]

Stenatopia have you checked the size of each bolt? May be some top cover drag going on check the bolt size and top cover gap. Misaligned recoil rod through hole? Plenty to go on but chamber is not it

 

[StenAtopia]

Stenatopia have you checked the size of each bolt? May be some top cover drag going on check the bolt size and top cover gap. Misaligned recoil rod through hole? Plenty to go on but chamber is not it

Good info. No drag from top cover. Bolts same size. Maybe if anything .010 difference. Now recoil rod opening there was a difference. The one bolt is 9.2mm opening. The faster bolt in claim( faster rof ,1/2oz lighter) the opening on that is 9.55mm opening. They both have a good amount of play (spring space) and charge super easily.

 

[StenAtopia]

Would a larger diameter spring/recoil rod opening increase the ROF?

 

[arch stanton]

Would a larger diameter spring/recoil rod opening increase the ROF?

I suppose if its not touching anything when in pogo stick mode.
Sight down the recoil rod holes or locate them and see if they are crooked or at a slightly angle in all directions.

 

[arch stanton]

Are you using the same recoil rod and spring? If not could be coil count making it run faster

 

[StenAtopia]

I suppose if its not touching anything when in pogo stick mode.
Sight down the recoil rod holes or locate them and see if they are crooked or at a slightly angle in all directions.

Both bolts holes are straight. Same recoil rod assembly for both bolts.(I only have one) Stumped. Driving me crazy. I know you say it’s not the barrel. Hoping it is. lol. Thank you as well for the replies. The only thing that was changed from the first outing was the bolt and barrel.

 

[arch stanton]

Next time out start with factory loaded cartridges and go from there. Reloads vary and cheap cartridges vary as well with ROF.

 

[AKarms]

Would a larger diameter spring/recoil rod opening increase the ROF?

If you have any drag on the recoil rod and spring it will slow the bolt. I have 2 micro uzis side by side; one cocks a lot smoother than the other. It runs slightly faster. There are no other differences between the 2. There is a slight drag on the recoil rod on 1, due to hole diameter

 

[StenAtopia]

If you have any drag on the recoil rod and spring it will slow the bolt. I have 2 micro uzis side by side; one cocks a lot smoother than the other. It runs slightly faster. There are no other differences between the 2. There is a slight drag on the recoil rod on 1, due to hole diameter

So you think that’s the reason one bolt runs 150-200rpm faster? Cause one recoil rod hole is bIgger than the other. Hmmmm interesting.

 

[Caverpete]

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.

 

[StenAtopia]

Just measured the chamber of the barrel in question. I know it’s tapered. So the deepest part before the step is .386 towards the mouth is .404

 

[AKarms]

So you think that’s the reason one bolt runs 150-200rpm faster? Cause one recoil rod hole is bIgger than the other. Hmmmm interesting.

Not saying it’s the entire reason. But if one feels smoother while racking than the other, I’d say it would have something to do with it. Also, same buffer? If not what’s the coil count on the springs. Could be different

 

[StenAtopia]

Not saying it’s the entire reason. But if one feels smoother while racking than the other, I’d say it would have something to do with it. Also, same buffer? If not what’s the coil count on the springs. Could be different

Same recoil spring on either bolt. Need to get more I guess. Yes same buffer. Thanks. Coil count I think is 67-69. I think I’m going blind trying to count the coils. Lol.

 

[shiklg]

Same recoil spring on either bolt. Need to get more I guess. Yes same buffer. Thanks. Coil count I think is 67-69. I think I’m going blind trying to count the coils. Lol.

that's not why you are going blind, LOL