Chamber fluting Cetme and H&K barrels with a 3 axis CNC mill
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  1. #1
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    Default Chamber fluting Cetme and H&K barrels with a 3 axis CNC mill

    I've occasionally used my home made CNC mill as a "pull shaper" since around 2010 to cut grooves inside the chambers of barrels I was making for my Cetme projects. I thought I was was done with chamber fluting for good a couple of years ago but needed to set the machine up one more time after getting stuck with a shitty "match" chamber by a current manufacturer of SR9-T "Match" barrels. There aren't any places that will chamber flute a barrel for an individual so maybe there's enough info in the video to help someone else.


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    I've got to ask, as I'm not familiar with chamber fluting, what is the reason for doing it? Ease of extraction?
    Doesn't it put some funky stresses on the brass when the pressure forces the brass into those grooves or at least reduce the reusable life of the brass?
    How about providing another venue for gasses to escape back in to the breech?

    Very curious.

    Thanks,

    -Ron

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    Quote Originally Posted by MetalCarnage View Post
    I've got to ask, as I'm not familiar with chamber fluting, what is the reason for doing it? Ease of extraction?
    Doesn't it put some funky stresses on the brass when the pressure forces the brass into those grooves or at least reduce the reusable life of the brass?
    How about providing another venue for gasses to escape back in to the breech?

    Very curious.

    Thanks,

    -Ron
    Random description from the inter web:

    "The Vorgrimmler action upon which HK based their roller locked actions predates the AR15 by several decades. The gas piston upper which is the current rage in the AR15 was abandoned by Eugene Stoner for a Direct Impingement principle in his design of the AR15. Almost simultaneously, in the AR18, the gas piston principle was incorporated. That the circle has closed, and the piston become popular again, has been capitalized upon by the folks at HK who have apparently taken the Stoner design and refined it to a point of great reliability. Add a piston and improve the magazine design and now you have a better mousetrap.

    The AR Gas piston upper is based on an entirely different operating principle: the venting of gases from the barrel to impinge on a piston which unlocks the action, and causes the weapon to cycle. The Vorgrimmler action is sort of a physics lesson in the subject area of inertia, which instead relies upon a retarded blowback impulse to cycle the action.

    The chamber fluting is an accession to the Vorgrimmler action. It is not necessary or even desirable in the Stoner action. Fluting exists in part because there are no primary extraction forces available in the retarded blowback arrangement, whereas in the Stoner action, the initial camming provided by the bolt carrier striking the action pin and rotating the bolt, frees the spent cartridge from its adhesion to the chamber walls as a vital aid to extraction. Due to this primary extraction in the Stoner design the cartridge is already free of the chamber walls before it begins to be extracted from the chamber.

    By contrast, in the Vorgrimmler design, the flutes extend down maybe 80% of the chamber walls thus floating 80% of the cartridge casing in an envelope of gas, preventing the case from sticking to the chamber walls, while the remaining 20% of the cartridge is able to obdurate and seal the chamber from additional escaping high pressure gas. The 20% sealing the chamber is easily enough overcome by the extraction forces at work in this design without damage to the casing. Without the fluting, one would experience failures to extract, and torn extractor rims from cartridges, if the Vorgrimmler action cycled at all.

    (As an aside, the initial development push by J.T. Thompson and the others at AutoOrdnance used the Blish lock to attempt to achieve the same retarded blowback action for the then standard Government cartridge of 30-06. I have always wondered whether their design would have worked had they tried fluting the chamber in that design.)

    To answer your question on how the next round is fed into the chamber, the opening and cycling of the action already having occurred in either action design, the next round is fed into the chamber similarly: the bolt strips the next cartridge from the magazine on its forward journey into battery."

    If you don't need those flutes, You Don't want them.!

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    Interesting. Thanks much Cal.

    -Ron

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    Yes, it's a roller delayed blowback action. The blowback force comes directly from the cartridge case (under pressure) pushing against the bolt face. The extractor does nothing more than hold the cartridge to the bolt face long enough for the ejector to kick it out. The "delay" is set by the locking piece angle providing leverage to overcome a spring under the bolt retaining lever. I've been doing a lot of experimentation with locking piece angles lately, mostly for suppressor use.

    The brass (and chamber)does get filthy and soft brass will usually end up with small bulges where it extruded slightly into the chamber flutes. It is still very reloadable, but it's ugly.

    I built one Cetme in .243 Win without chamber flutes and instead copied an SVT40 short stroke gas system. The excercise was kind of pointless (in retrospect) but I learned a lot and others have used the information to make their own frankenbuilds. The most notable is an MP40sorta in 8mm Kurtz using HK33 and FAL gas system parts.
    Last edited by Holescreek; 10-23-2018 at 05:17 PM. Reason: additional info

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    So if you reload, this ain't a good idea?

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    Quote Originally Posted by Butch Lambert View Post
    So if you reload, this ain't a good idea?
    I reload all the fired brass from my fluted chambers. Before I quit counting some of it had been reloaded 12 times, probably more now. The cases look like crap because of the lines and I only shoot them again out of fluted chambers but the accuracy doesn't seem to be affected at all by the deformities in the case. My best rifle will shoot .8" groups at 100 yards with reloads, not horrible for this kind of rifle.

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    Quote Originally Posted by Holescreek View Post
    I reload all the fired brass from my fluted chambers. Before I quit counting some of it had been reloaded 12 times, probably more now. The cases look like crap because of the lines and I only shoot them again out of fluted chambers but the accuracy doesn't seem to be affected at all by the deformities in the case. My best rifle will shoot .8" groups at 100 yards with reloads, not horrible for this kind of rifle.
    Do you find the brass "seals up" well enough? There is not much of a full contact band between the flutes and the base.

    I would love to see those groups. One shot or two? ;-)

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    Quote Originally Posted by CalG View Post
    Do you find the brass "seals up" well enough? There is not much of a full contact band between the flutes and the base.

    I would love to see those groups. One shot or two? ;-)
    That's a great question! I was only looking for cracks in the cases and didn't think to chronograph them for comparison to first time fired cases. I don't have target pics here at work so they'll be forthcoming. I do have access to a pic of the rifle though, I finished building it in 2012. I patterned it off of a couple of museum photos I found on the web of the first H&K PSG1 prototype. This one has Cetme internals and the barrel is a McGowen blank I profiled and chamber fluted. I pulled the barrel off in January 2017 and threaded it for my suppressor.

    Attached Thumbnails Attached Thumbnails 2v2jq7ppmxajwmd.jpg  

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    Quote Originally Posted by Holescreek View Post
    That's a great question! I was only looking for cracks in the cases and didn't think to chronograph them for comparison to first time fired cases. I don't have target pics here at work so they'll be forthcoming. I do have access to a pic of the rifle though, I finished building it in 2012. I patterned it off of a couple of museum photos I found on the web of the first H&K PSG1 prototype. This one has Cetme internals and the barrel is a McGowen blank I profiled and chamber fluted. I pulled the barrel off in January 2017 and threaded it for my suppressor.

    I don't really want to see holes in paper. It was a ;-) and a celebration of accuracy.

    On the sealing aspect, MV would be less indicative (to me) than a dirty bolt and action, ejector, brass etc, after just a few rounds.

    Relative for sure....

    Blow backs must run dirty by nature.

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    Yeah, you can't go by dirt in these things. They are filthy on a good day. I made another video from testing different locking piece angles for shooting suppressed. I used a high speed camera to catch the bolt movement and the cloud of smoke ejected with the cartridge is horrible with the suppressor attached.


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