8mm groove diameter

[prewar47]

My Merkel combination gun is chambered for the 8x57 and is marked "8 J.R." I know that this implies a .318" boring but a slug measures .3204" and a .308" minus gage pin can just be inserted into the muzzle.

Did any of the firearms makers barrel so that either .318" or .323" ammo could be shot in it or is this simply an anomalous Merkel barrel? I already swage 8 mm bullets so that is not a problem to get a correct bullet diameter but the factory Norma 8x57 IRS ammo is really tempting so the second question would be does anyone know of actual controlled test data evaluating pressure rise with a bullet that is .0026" too large?
Thanks,
Dennis

[Axel E]

Dennis, prior to the 1940 proof law, there was no definite distinction between 8mm I (.318) and 8mmS (.323") barrals. In the 1920s and 30s many compromise barrels were made, allowing the use of both I and S factory loads. At that time, sporting I cartridges were usually available with a larger bullet selection, especially above 200 gr.
Contrary to popular belief, it was not the too tight barrel bores that made shooting S loads in I barrels dangerous, but chamber neck and throat dimensions. Not only the bullets, but the case necks too of the S cartridges are .005" fatter than the I loads. As smaller gunmakers resharpened and used their expensive chambering reamers as long as a cartridge still slipped into the chamber, many 8mmI chamber necks and throats were undersized even for I cartridges. If an S cartridge was stuffed into such an undersized chamber, there was no space for the case neck to expand and release the bullet and the "oversize" bullet was wedged into the undersize throat. Three quotes from 1930s handbooks and a factory dimension table:
RWS: "There never was a problem with modern (read 98 action) repeating rifles. On break – open rifles, especially drillings with their thin rifle barrel walls, the chamber dimensions must be checked before using S ammo."
DWM: " If you want to shoot S ammo from your rifle, have the chamber checked and enlarged, if necessary, by your gunsmith."
Mauser, Oberndorf: " Our hunting rifles in 8x57 and 8x60 (bore diameters given as .309" land, .319" groove) get the chamber and throat for S bullets, so such loads may be used as well."
So check the chamber of your Merkel this way: Shoot an I (318") load. If an S (.323") bullet slips int and out the fired case, your rifle is ok for using S bullets. Check your seating depth too. A S bullet should have about .15" free movevement before it engages the rifling.

[mike ford]

Dennis,
Axel is precisely correct and I suggest that if a .323" bullet will not enter the neck, you should try a .321" 32 Win. Special bullet. I have seen good reviews of the new "Flextip" bullets, which are made for the 32 Win. also.
Mike

[prewar47]

Great information, Axel. Just what I was looking for.

I have a 1937 Merkel 8x57 IR double and its bores were also around .320". I have not fired the combination gun yet but a .323" bullet will not enter the neck of cases fired in the double rifle so at least that answers the question on that one.

On the combination gun a .323" loaded round will not drop fully into its chamber and probably would require a bit of force to close the action on the round. Also, a .323" unsized fired case requires force to close the action so I'm pretty sure we know the answer on that one too. I have Sellier and Bellot IR ammo and if it shoots well in the combination's .320" bore I'll just be satisfied with where I'm at.

Regarding your DWM quote about enlarging the chamber to accommodate the IRS ammo, do you know if this was a common practice? I know most of the major reamer makers can provide neck and/or throat reamers and that would be one fix if IRS ammo was desired.

Thanks for your thoughtful response.

Mike--I have a lot of .323 bullets and have a .319" swaging die so am pretty much set in that regard but thanks for the tip on the 32's.
Dennis

[mike ford]

Dennis,
Of course I may be mistaken, but I seem to remember that before announcing the conversion from .318" bullets to .323 and issuing the ammunition, the German government inspected all 8mm rifles in their inventory( incl M88s) to insure they could be used with the new ammo. In addition to changing the rear sight scales, they inspected the neck/throat area of the chambers and opened up the necks if necessary. It is important to understand that this was not done with sporting guns, nor with rifles that had been provided/sold to other nations. The careful application of a normal 8x57IRS reamer should accomplish what you are suggesting. However, factory 8x57IRS ammo may or may not group well in the 1937 Merkel DR, depending on which bullet it was regulated for. It that case it would be a hand loading project and you might just as well load 8x57IR with the correct bullet( my 8x57 IR rifles do well with the .318 196gr bullets, but none are doubles, so I don't know).
Mike

[Diz]

I think you may be better served by handloading ammo for this double as Mike suggests. Anytime ammo causes difficult closing I generally suspect either a tight neck clearance or a long bullet. A bullet seated against the rifling will raise pressures a bit but a tight fitting neck that delays bullet release can raise pressures a lot. In my opinion I would do a chamber cast of both sides, long enough to pick up some of the rifling and check all the critical dimensions, then load accordingly. Thanks, Diz

[prewar47]

Thanks for the input Mike and Diz. I shot the gun yesterday with the S&B factory ammo and the inside case diameter is a measured .322" using a minus gage pin so the .323" bullets are definitely a no-go for both of the guns. The combination gun weighs 6 lb 9 oz and that is really nice as a carrying weight. This weight will be nice for wingshooting as a shotgun and it fits me superbly but as a rifle it is a bit on the unpleasant side for shooting from the bench. The Zeiss scope discussed in another thread would add about a pound and tame it a bit.

Diz--I will do chamber castings of the DR as well as the combination gun. They were made within a few years of each other and would be interesting to compare them as well as have the information available that you suggest.

[Diz]

prewar47, I would be very interested in the results of the castings and I am sure there are others on this board including Mike that would like to know. If you could post a detailed drawing of the results that would be very helpful for all. Best of luck going forward with your projects. Thanks, Diz