Help from the Mauser Experts needed

[Patrick Chadwick]

"Correct" is not a word that you will find in standards documentation, but a somewhat subjective adjective, popular with collectors, that already implies a judgement.

The "Masstefeln für Handfeuerwaffen und Munition" - which is the legally binding set of dimensions that would be used by a proof house - gives the following
for the chambering 8x57JS:

Felddurchmesser (diameter across lands) 7.89mm
Zugdurchmesser (groove diameter) 8.20mm

These are minimum dimensions.

Information

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[Patrick Chadwick]

It seems that your bore is seriously worn. I have not yet found any official definition of maximum land and groove diameters. Until they turn up, let's look at it from a practical viewpoint.

"Shot out" is not an official definition that you will find in any set of standards. But for a specific rifle it may be defined as that level of wear at which the rifle no longer performs satisfactorily. There are perhaps 3 major factors. A badly worn barrel will usually have a combination of these factors.

1) Throat erosion. Results in the bullet having a free flight before it properly engages the lands. Since the cartridge neck has some play in the chamber, it sits on the bottom, and the bullet emerges in the lower portion of the space available. It is thus engraved on the skew and will tend to have a corkscrewing motion when it leaves the barrel. Depending on the amount of wear, this can result in anything in the spectrum enlarged grouping through keyholing to missing the target entirely at what would be considered point-blank ranges. Hence the general advice to keep the bullet seated as far up the throat as is sensible, and to avoid boat-tailed bullets in military rifles, as these rifles tend to have generous throats by comparison with modern standards. A standard test method is to drop in a bullet of your choice into the throat and measure how far it enters the barrel, i.e. to measure the overall cartridge length that would be required for the bullet to engage before it has left the neck of the cartridge If that position is beyond the max. OAL of the cartridge, then you have a problem. Mauser rifles generally have a very long throat, so you need to look for a bullet with a long cylindrical section. Don't forget that the designation 8x57JS stands for 8x57 cartridge for Infantry (the J) rifle using S (spitzer) bullets. Not boat-tails!

2) Enlarged land diameter. Reduces the engagement of the bullet in the rifling. No immediately disastrous effect until the stage is reached at which the bullet is no longer being grasped by the rifling and is stripped by the grooves without being rotated. With lead bullets this results in immediate serious leading of the barrel and disastrous grouping, as you are effectively firing a smooth-bore gun. I have not experienced this with jacketed bullets, but I have experienced it with copper-plated bullets, with recovered bullets showing that the copper had been stripped off the lead.

3) Enlarged groove diameter. Assuming that all other things are equal - which, in practice, they never are - this allows blow-by of hot gases and is a factor in increasing throat erosion. With jacketed bullets, the only remedy is to use a bullet that is as close to the groove diameter as possible or even a tiny bit (0.001") larger. This trick is limited by the available space for the cartridge neck in the chamber. With lead bullets you have more freedom, as they upset to some degree in the throat, so that even if the largest possible bullet is too small, you can gain a bit more obturation by using a slightly softer lead alloy.

Summarizing:
a) Measure the groove diameter.
b) Select a bullet that is equal to this diameter or + 0.001
c) Select a bullet that has a long cylindrical section, to keep the base guided by the cartridge neck as long as possible
d) Check that you can actually chamber a cartridge loaded with such a bullet.

I hope you can find a solution!

[Steve1152]

Patrick

Very late, but thanks very mcuh for your reply. I have to conclude the barrel is worn and needs replacing!!