Headspacing

[G.T.]

Hello to all! I'm not a gunsmith, hardly even a mechanic.. but I've always had a suspicion that the best way to learn, was to do? The second best way is to listen closely to those who did! Soooo, I'm going to relate a recent experience with re-barreling a Luger with a standard P.08 4" military take off barrel... excellent bore, and very nice overall... I removed the .30 Luger barrel without incident, good so far... Then I installed the used original 9mm 4" P.08, went on perfectly, so good so far....
Drop in the "GO' gauge, closes fine... Drop in the "NO GO" gauge and closes fine! What? Wait a minute, NOT GOOD! How come, What? Why?.... Need to dig further...

First observation, toggle assembly seems very loose?.. Most of the play is in the two forward pins, the breechblock pin and the middle toggle pin... I HATE to work on the middle toggle pin as sometimes the small middle toggle pin retaining pin becomes a work monster all by it's self! So, what to do, well the rear pin seems fine, but we'll put in a pin that is as new and fits pretty tight, then, we install a breechblock pin that tightens things up a few thousands as well? As we are being timid, we hope the middle toggle pin is good enough?... Re-test with the "NO GO" and find you have to push a little, but it will still snap closed on the "NO GO".. Good enough? Let's go the range and see!
First round, perfect! No issues, problems, works fine... Halfway thru the first mag full, I see one of the empties has a black primer hole? I load up another mag, as I'm determined to see if this is a problem or just a fluke? At the end of the second mag! Kaaa Boom! we got trouble! I could feel it, and the gun jammed... I cleared it and was ready to continue, and thought, stop you fool... pay attention?... So, knowing it needed improvement, home I went...
Well, the result was, on the brass I collected, one of the pierced primers had flattened the primer as well? Also, it broke the fining pin retainer head off, and pushed it thru the back of the breechblock!! Not only that, but it also lifted the breechblock metal on both sides of the extractor, failure was on its way, a few more rounds and we would have probably had a catastrophic failure...
Soooo, after a little thought, I replace the breechblock with a nice original, breechblock (at my expense, and also the firing pin retainer, also at my expense!) and totally re-pined all the of the toggle train with original oversize pins!!! Wala, head space came back to normal, (probably .004" over "GO"?) range test 100% normal...
Off and away back to the owner! I also replaced the solid firing pin with a fluted one, and would have liked to see if this would have happened at all with the later pin? But, I'm short on breechblocks and not really interested in ruining more, at least for now?... … BTW, the difference between the "GO" & "NO GO"
is approx. .020" I now think, if you have play in your chamber, it better not be more that .010" past "GO" or more than half way between the gauges?... Less is MUCH better, and very tight on the "GO" even better yet!!! Hope this enlightens some of you! No place for the timid here! Best to all, til...lat'r....GT....

[DonVoigt]

Saami min and max are 0.754 and 0.776 " for 9mm head space.

I don't believe headspace was at the max or no - go, if the toggle closed, it was "more"! JMHO.

More likely the ridge at the front of the chamber was/is worn and allows "undersize" diameter cartridges to go too far past the point where they should stop. The headspace gage has a sharp shoulder and is steel, if it closed there is something amiss in the chamber.

The cartridge with holed and flattened primer went past the chamber front, pushed by the striker/firing pin as far as it would go against the hook of the ejector; when it fired, the primer backed out-puncturing the prime, and then the cartridge was then forced back to the breech face- flattening the primer in the process.

I have convinced myself that drilling the "Norwegian" gas relief hole in the bottom of the breech block is a very good idea, and should be done on any re-barrel job, and probably to any shooter luger. Collectible lugers probably should not be modified and left in their original condition- since they don't get shot anyway. Gas escaping to the bottom may result in a "blown" magazine- another reason to use "mecgar" mags for shooting and not original mags. But mags are cheaper than a new bolt, extractor, and possibly other associated parts.

All JMHO. But I agree 100% that near minimum headspace is preferable to maximum.

[G.T.]

Hi Don, one has to remember that the current commercial loadings are about .010" under the "GO" gauge... So, you are sporting a .010" free space even when tight on the "GO"... Sooo, closing on the "NO GO" adds up to approx. .030" Plus some.... Waaay, to much.... The fact that it straightened out when the toggle train was corrected was an indicator of a good, although deep, chamber... The barrel was obviously chambered at the factor for a long tolerance toggle train, and the switch didn't allow for this?... It is normal on the ridge, and nothing is going past? A lot to digest? And of course, I'm not positive of my theory... Best to you Don, til...lat'r....GT

[G.T.]

The hole is drilled on the machined recess of the hold open relief on the bottom of the breechblock?... Not very big either?... probably 3MM give or take a few.... GT

[Ron Wood]

Here is a modified breech block, Not sure if it is Norwegian or Finnish?

[Dick Herman]

This pressure relief modification is found on many Finnish m23 Lugers. Did the Norwegians make the same modification?

[G.T.]

That would probably work even if the pin is not relieved?.... GT

[Ron Wood]

Yep.
Ron

[Mark1]

GT,
I have encountered the same or similar problems many times in the past. The issue is with some particular calibres or types of cartridge case and is due to the very large chamber dimension tolerances of the SAAMI specification. The solution is to use the much closer CIP (European proof and ammunition standards organisation) specification headspace gauges for all chamber work.
For example, the SAAMI spec. headspace tolerance range (Go-NoGo) for the 9mm Luger is .022" whereas the CIP range is .012". The same applies to the 45acp cartridge, SAAMI .022", CIP .012". For most rimless rifle cartridges, SAAMI headspace gauges will give a range of .010", the CIP gauges are .004".
I have no idea why there is such a large variation between the 2 standards, but for me a lot of the problem guns disappeared when I began using the CIP gauges. The best source of CIP gauges I have found is Pacific Tool & Gauge there in the United States.
Mark