really weird chambers
 

i'm sure several of you have noticed this but... In the blueprints for original parabellum barrels (9mm) you'll notice a small shoulder in the middle of the chamber, I didn't encounter this in my friend's wwii parabellum, but my 1915 sure has it. My empties come out with an unmistakable bottlenecked effect. What on earth is the purpose of this shoulder, if any???
:confused:

Hi,

The normal shape of the 9mm cartridge is a slightly tapered one. For a decent fitting, the chamber therefore should also be tapered.

It proved to be easier/cheaper/quicker those days to create a chamber with a stepped straight-edge design. This ensures correct chambering and produces the ring mark as a result.

Good quality cases will only show scuff marks, thin-walled cases will actually show a bulge.

Wachsbaer,

In addition, due to wear or production variation, the 'step' will be found more or less prominent (up to hardly apparent at all), and sometimes will not appear completely circumferential.

--Dwight

My barrel is in pristine condition, therefore the obvious mark I suppose. I got to thinking while I was at work today that it probably had something to do with the fact that the cartridge is tapered, and it was cheaper to just step the chamber, and you confiremed this above. I always know my brass even when I miss a piece and find it months later, as I'm the only one around here with such a gun, it seems. I wonder if this odd "quick fix" helps any in extraction, sorta like a fluted chamber or whatnot... hmmm. Thanks, Tom

Hi Tom,

It does, as the stepped design allows for effective sealing of the front portion of the chamber. It offers the same sealing ability as on bottle-necked cartridges and prevents gases from escaping past the neck of the cartrigde.

So less fouling, less chance of the cartrigde getting stuck, thus better and more reliable extraction.

Early experimental versions of the 9 mm cartridge had a slight bottle neck.

My theory is the step was to accomodate the bottle neck and they didn't bother to change it after going to a straight tapered cartridge.

I don't think my theory is worth a plugged nickel.

On the other hand a tapered reamer would be no great trick, so I don't think it would really be easier to make a stepped chamber.

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva"> On the other hand a tapered reamer would be no great trick, so I don't think it would really be easier to make a stepped chamber. </font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">No, a tapered reamer would be no great trick... but straight reamers in various sizes would be a "standard" that could be produced more economically...where a specific taper reamer would be custom I think the design is based on the manufacturing tool cost... And if something isn't broken... don't try to fix it... :D

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by G. van Vlimmeren:
<strong>Hi Tom,

It does, as the stepped design allows for effective sealing of the front portion of the chamber. It offers the same sealing ability as on bottle-necked cartridges and prevents gases from escaping past the neck of the cartrigde.

So less fouling, less chance of the cartrigde getting stuck, thus better and more reliable extraction.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">I agree completely with the sealing theory, and when using slower burning powders, this step does seal the chamber quite effectively.

Both my 1917 DWM and my 1941 byf have the step in the chamber, but my 1970s' Mauser Parabellums do not have the step.

It appears that the chamber design was well thought out and executed in production. Now if I could only figure out why .358 grooves were used with .354 projectiles?!?

Sieger

There is still a taper in the stepped chamber. At least that's how it looks in the chamber picture (I am refering to the 9mmbarrelstub.jpg image that was posted on this site).

I'm wondering if the earlier ammunition was slightly different.

Then the H&K P7 also uses the chamber ring (behind the flutes)as a gas seal?

Hi,

Gortz stated that Georg Luger patented the design in 1910. So the change was considered important enough to warrant a patent.

Most probably to cure some problems with early, less reliable smokeless powder, so Dean, I guess you're right.

Sieger,you asked why .358 inch grooves were used with .354 inch projectiles.

My Luger (1918 DWM) is stamped 8.82mm (.348 inch) on barrel, which I believe indicates diameter across lands. If so, there would be .354-.348=.006 inch diametral engagement in grooves. Isn't that OK? Are you thinking the displaced metal should completely fill the grooves? Wouldn't that be a poor idea in view of inevitable tolerance ranges and possibility of jamming?

The usual practice is to make a jacketed bullet the same diameter as the groove diameter in the barrel. In the case of both the 7.65 and 9 mm Lugers the groove diameter is larger than the bullet diameter. (Even allowing for tolerances.) This leaves the question, is the bullet undersized or is the barrel over sized? Based on a few other factors we are inclined to say the bullet is the expected diameter and the barrel is over sized. This for some reason we have not yet determined although we have some half baked theories.

The marking found on some 9 mm barrels indicates the land to land diameter of that specific barrel.