Chamber fluting Cetme and H&K barrels with a home made 4 axis CNC Mill

[Holescreek]

I've been occasionally using my little Rong Fu 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 and decided it was time to make a video for future generations.

 

[Tozguy]

Hi Holescreek,
Thanks for the video. Would you mind explaining what the purpose of grooving the chamber is. Also, does it complicate reloading of the brass in any way.
I have seen some fired cases from a fluted chamber at the range but never knew anything about it. Thanks.

 

[JBuckley13f]

The chamber fluting allows gas to flow around the case to help extract the case under high pressure.

 

[gi_984]

EDIT: I don't remember using any arsenal produced CETME, so unfamiliar with how their barrels are made.

Interesting. Have used a lot of H&K (mostly G-3s and MP-5s (K, SD, etc) while in the military. Both stateside and overseas. Even used the Paki licensed stuff while working in Pakistan. But they were all original arsenal produced weapons. Believe they were all hammer forged barrels. Not sure what H&K is currently using for re-barrelling.
Tozguy: I've reloaded lots of range pick ups from civilian H&K 91s and 93s. Back in the day when brass was hard to come by locally, the fluted stuff was all the range scavengers would leave on the ground. The locals thought it was unfit for reloading.

Holes creek: Nice documentation with how you cut the fluting. How much polishing does the chambers require? Any issues with extraction?

 

[Holescreek]

All HK/Cetme rifles are delayed blowback actions that require the cartridge to push backwards on the bolt face to overcome the roller delay system and cycle the action. Without chamber flutes the gas would expand inside the chamber and lock the system up. The flutes allow gas to surround the case externally and equalize the pressure so the cartridge will push backwards freely.

The brass does get mauled up some depending on how deep and wide the flutes are cut. If the flutes are too shallow or narrow the gas can't do it's job. If they are too wide or too deep the brass will expand into the flutes and lock the cartridge in the chamber.

I have reloaded some brass over a dozen times. Lots of case lube helps on the worse ones.

 

[cjtoombs]

Was wondering if you used a fixture to set the tool in the toolholder. Looks like the tip placement would need to be very consistent between tool changes.

 

[Holescreek]

Was wondering if you used a fixture to set the tool in the toolholder. Looks like the tip placement would need to be very consistent between tool changes.

I remove the tool holder (just the horizontal part) from the spindle at every tool change and set the new carbide tool perpendicular to it by eye and set the height to .07" with the tail end of a dial caliper. Then the set screw gets tightened and I grind off any excess carbide sticking out of the top of the tool holder on a diamond wheel. I grind my carbide bits on a Deckel SO grinder with a diamond wheel.

So far I've never had a crash inside a bore, but it is still a little unnerving every time I send a new tool bit into the chamber. There's no way to see what's going on until it's finished so I just had to learn to trust the numbers.

 

[Newguy]

Hi do you do the The chamber Fluting for anyone else. I’m want to do a Msg90 clone they only have a psg1 type barrel and it’s not hammer forged. Lother Walter has the proper Barrel with a target weight at the muzzle. And it’s hammer forged.is it possible you can flute the chamber for me. I’m not looking for you to do it for nothing let me know what you want. Your my last hope.