I wrote to the German Gun Collectors Assoc. and got two near instant and detailed replies from two serious experts, one the author of the definitive book on Original Oberndorf Mauser Sporters...what great fellows they are to take the time to do this for me...Note that it really doesn't have much to do with bore diameter...but instead is a problem with the case neck area in the chamber...see below:
Hello Mr.Peck,
Dietrich Apel asked me to repond directly to your questions
regarding bore sizes etc. with your 1914 period Type M carbine in 8,0 N
(Normal). In most cases the term N for Normal indicates the smaller .318
bullet diameter. Your pre WW 2 Type M carbine has the typical
bore/groove
dimensions of 7.85, 8,15 with 360 twist. The numbers 156,14 indicate the
actual Bore Guage number for the 8X57 bore bullet of this period. In my
Mauser sporter book is a chart that shows all the bore/groove/rate of
twist
etc for various cartridges. This was published around 1922. On the
bottom of
this chart in finer print is some critical information regarding all the
8mm
rounds. It states that after WW 2 all barrels or barreled actions
actions
had their chamber necks increased so as the larger .323 diameter bullets
could be fired in these chambers without any pressure build up. In fact,
contrary to all the published liturature before my book came out
indicated
it was NOT safe to fire the .323 bullet out of a .318 marked barrel. It
is
NOT the bullet diameter difference that can cause pressure problems but
the
fact that the .323 cartridge neck area was larger due to the Total
diameter
of this part of the case to take the larger .323 S bullet. If you take
the
bolt out of your carbine and remove the firing pin and sleeve unit then
you
can safely put a 8X57 S round in the magazine , put the bolt back in the
action(With firing pin removed)and slowly load the round into the
chamber.
If the round goes in tight you know your case NECK area has NOT been
opened
up so in this case it would NOT be safe to fire the larger bullet!! If
the
round goes in easily without extra effort it would indicate that the
neck
was opend by someone at a later date etc. If you wish to follow this
procedure you should only test fire a round by the use of a string on
the
trigger and yourself some distance back and to the left of the carbine
when
you fire it. If all goes well you should be able to open the bolt
Without
any extra effort. If this happens then you can safely shoot .323 S type
rounds out of this carbine forever!!! If you have any other questions
please
E mail me ! REGARDS, JON SPEED
Hello Robin,
8x57 and bore diameters - a classic German rifle dilemma! Dozens of articles have appeared in publications such as Rifle and American Rifleman magazines and whole sections of books have been devoted to this problem and there's still no simple answer. Your rifle is a good case in point. Yes, it's true that the German military adopted the larger "S" diameter in 1905, but many German sporting manufacturers clung to the nominal .318" "J" bore for quite a while, not fully accepting the "S" bore as standard until late in the 1930s. It's also true that Many German sporting gunmakers preferred slightly undersized bores compared to our way of precisely matching bullet diameter to groove diameter.
If I had your rifle, the first thing I would do is slug the barrel. All the numbers on the outside are great, but until you can actually measure it you won't know for sure. If it does turn out to have a .320-.321" groove diameter you will probably find that .323" bullets will work fine if you are careful to watch for pressure signs as you work up loads for it. There are also 170 grain .320" diameter bullets available from Hornady and Speer designed for the .32 Winchester Special cartridge that work well if you keep their intended velocities in mind. I have used these bullets in a couple of 'in between' 8mms with very satisfactory results. One last thought for an 'in between' size bore would be to get a bullet reducing die from Richard Corbin. With such a reducing die you could squeeze just about any .323" bullet to .320" or .321" easily and load them from there.
The '156,14' mark is actually the gauge of the bore. The British had set up standard gauge sizes from .300" (172.28 gauge) to .450" (51.05 gauge) in .010" increments in the same sense that a shotgun bore is gauged, by the number of round lead balls of that diameter that make a pound. The Germans adopted this somewhat awkward system as well, and the gauge number marked is the largest size that would fit in the bore. In your rifle's case, 156.14 is the standard gauge for .310", so the bore (not groove) diameter when proofed was somewhere between .310" and .319".
All in all not a very definitive answer, I know, but each one of these 8mm rifles seems to be a rule unto itself.
Steve Meyer
German Gun Collectors Association
