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View Full Version : Cetme C actual headspace.



Daniel
03-02-2018, 07:43 PM
First off I know that most people consider headspace on a cetme to be the bolt/carrier gap, I am however refering to actual headspace.
So my question is, I have a pair of ptr "match" barrels (https://www.hkparts.net/shop/pc/PTR-18-HK-G3-HK-91-Match-Grade-Barrel-9p873.htm) and a set of cetme bolts, just for kicks I hooked 308 go-gauge into a clean bolt head and incerted it into a clean chamber, with firm pressure I was unable to get either of the bolts to close on either of the barrels. I used a feeler gauge and have .005 gap on both barrels/bolts with the go-gauge. I have only headspaced bolt action rifles where this would be a problem so it's got me curious, is this normal for a cetme/hk9x or do I need to send these back to get chambered correctly?

holescreek
03-02-2018, 10:42 PM
Those are correct.

True head space on these guns is established in the chamber reaming process and the depth is set based on the amount of cartridge sticking out of the finished chamber. A better way of measuring (that doesn't involve trying to hold a bolt head steady while you use a feeler gauge) is to hold the barrel vertically and insert the go gauge, then measure from the rear of the gauge to the barrel face. Then measure the depth of the counterbore in your bolt face. You should end up with around .005"~.009" more cartridge exposure than bolt depth. The reason behind the gap is to keep the bolt face from striking the rear of the barrel every time the weapon is fired.

The bolt gap we all use has nothing at all to do with headspace. The bolt gap merely determines where the bolt locking pawl from the carrier rests on the ramp of the bolt head. A gap under .004" means the pawl will be too far forward of the ramp and over .02" the pawl will be too far up the ramp. Since the spring pressure from the pawl is what actually controls the delay of the bolt opening it's important to control it's location.

Daniel
03-05-2018, 10:33 PM
Thanks again Holescreek, thats exactly the answer I needed. I'm really trying to do this right and take my time since it's my first build of this type so there may be more silly questions before I'm done.

holescreek
03-05-2018, 11:43 PM
It will be interesting to see what PTR Match barrels are capable of. Post the build here so we can follow along!

Here is one of my Cetmes:


https://www.youtube.com/watch?v=DgZJcbuvc8Y

Daniel
03-06-2018, 09:18 AM
I saw that video. Definitely an interesting build. Did you find a left hand cocking tube or just cut a blank to get your upward unlocking action?

holescreek
03-06-2018, 10:15 AM
I make my own cocking tubes.

When I first started building these things I had a thread up that showed how you can reverse the direction of a standard tube. You make a copper heat sink to slip fit into the back of the tube you can cut a new notch opposite then weld a plate in the upper notch. You have to do something similar with the pin holes to be able to get the carrier pin in and out.

Daniel
03-06-2018, 01:21 PM
Makes sense, thats a good idea. Should I post progress here or in a new thread?

holescreek
03-06-2018, 03:21 PM
You should start a new build thread.