fluted chamber + open bolt = success or disaster?

amphibian

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Some of you may have seen this project I have been working on with my hybrid CMMG RDB (Radially Delayed Blowback) 9mm Dissent variant to make it full auto capable and tune-able which has been a huge success for me: https://uzitalk.com/forums/index.ph...l-auto-cmmg-rdb-fixed-ejector-project.100045/

Direct link on my site: https://c3junkie.com/?page_id=1733

It has been running great but of course I can't leave anything alone so I am tinkering with modifying a perfectly good brand new RCM CHF MP5K fluted barrel for use in this setup:
MP5K-PhaseII_1.jpg

MP5K-PhaseII_2.jpg
I tested it this AM and.....
MP5K-PhaseII_brass2.jpg

The barrel OD is so small on the MP5 that I was worried that I wouldn't have enough shoulder for the extension so I purposely left it a little long and had some light hits. So initial testing is positive but I need to pull the barrel and face the shoulder some to tighten up the headspacing for reliable operation. Then I can hopefully get some FA cyclic rate results.

So I have also been wondering what would happen if I took a fluted MP5 barrel and turned it down to work in an UZI. Full size MP5 barrel is 8.9" and my full size UZI barrels that are 3 lugged are right at 9". So I think I could make a threaded sleeve to replicate the flange of an UZI barrel so I could put that barrel in an UZI.

Obviously none of this 'needed' and I would argue that the MP5 does NOT 'need' a fluted chamber either. One of my friends supports guns for a MG rental range and swaps their rental MP5 barrels with cheap blanks he cuts on his lathe with NO flutes and those beat up rental guns run like a champ.

This is for educational / entertainment purposes but it is interesting that in my limited testing with my hybrid CMMG RDB dissent, it runs with NO extractor installed in the bolt.

It would be really interesting if it would work in an UZI and if someone had the means to do this, you could theoretically not need an extractor in your bolt assuming your ammo goes bang...obviously you will need it to clear dud or something. Or if you have a worn out extractor, it wouldn't have to work too hard.

Or is this a bad idea and using a fluted chamber in an open bolt SMG is dumb? Anyone ever try this or see this done before?
 
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amphibian

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Any effect on cyclic rate?
I guess you didn't read what I put after the last picture:
The barrel OD is so small on the MP5 that I was worried that I wouldn't have enough shoulder for the extension so I purposely left it a little long and had some light hits. So initial testing is positive but I need to pull the barrel and face the shoulder some to tighten up the headspacing for reliable operation. Then I can hopefully get some FA cyclic rate results.
 

ericthered

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I dont have a direct answer to your question. While not being a authority on the subject, I would guess that it will probably have the faster more assisted extraction you mentioned, which will lead to both a slightly faster ROF and quite a bit more gas at the port. I wouldnt think there would be any more of a safety concern then is standard in slicing and dicing guns together.

I know you always do excellent test work, so that will be interesting to see. If you were able to compare open and closed bolt that would be cool.

The only thing I dont like about the cmmg setup is the wiping of the brass with the twist motion. I dont know if it would actually make any functional difference in that aspect. I would be curious if you end up with a shavings collection after a while.
 

amphibian

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Very nice Amphibian you are on to something. Th PK machine gun I believe is fluted and open bolt. Very interesting for an Uzi set up
Are you sure on that? I tried to Google that and see that the PK is fluted on the outer contour but not the chamber. Do you have any links you can reference? Thanks!
 

Chef

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I have an old (NOS) Urbach Mp5 barrel and it's not fluted. So I presume the flutes were not 100% necessary with the Mp5.
 

arch stanton

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Are you sure on that? I tried to Google that and see that the PK is fluted on the outer contour but not the chamber. Do you have any links you can reference? Thanks!
Figured I could get you pic but that barrel is not around here anymore. From what I am reading now it looks like it was a partially fluted barrel.
 

ericthered

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Very interesting. Thank you for the update! It seems that with the cmmg setup it would be harder to verify if the rof decrease is from drag. Just because of the non backward compatibility. If you do get the chance to test it with a direct blowback gun it should give you some data on what is happening. Or I guess you could ask your mp5 rental buddy if he has measured rof difference in his roller delay guns as he uses both barrels.
 

amphibian

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Or I guess you could ask your mp5 rental buddy if he has measured rof difference in his roller delay guns as he uses both barrels.
I thought about that too but he is an old timer that doesn't give a shit about cyclic rate, or have a timer. Same goes for the people at the rental range. They don't care and I have never tried to time a SMG in an indoor range and plus deal with all the other people present.
 

ericthered

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Ah. Yeah, it would be tough in a indoor range.
Outside of spending money. Since you have some access to a lathe at least (guessing from your first post). You could make a inverted barrel bushing to screw the mp5 barrel into that would allow it to be used with a standard blowback AR bolt. Eyeballing sizes it looks like you should have enough clearances to fit it together. Should be able to get the rear chamber face flush.
 

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