Peter

Lets not get too carried away with bullet diameters. There is plenty of information around about the different bore diameters of the old German and British DG rifles, with those for the .404J and 10.75x68 Mauser cartridges being no exception.

I have an original Type A Oberndorf Mauser opened up from 10.75x68 to .404J as many of the British gunmakers did with the standard length Military or Oberndorf sporter actions. Obviously my barrel is the 10,45/10,75 groove and bore as mentioned by grandveneur above (these are the actual measurements stamped on my barrel too).

As for projectiles, I use original RWS 347gr flat point soft nosed bullets and RWS 400gr FMJ's in my reloads as well as original Kynoch 400gr soft point ammunition and Parker Hale (loaded by Norma) 400gr FMJ factory ammunition. All engrave in the rifling and shoot well on target and animals. Other projectile samples I have all measure between .421" and .423" and I am sure would all shoot very well in my 10.75x68/404J.
350gr and 400gr gas checked cast projectiles sized at .425" also shoot extremely well.

There never was a 400gr factory load for the 10.75x68. All things being equal, it is ballistically impossible to match the .404J using a smaller case especially if maintaining reasonable DG working pressures and bullets seated to normal depth with a crimp applied. Experimental loads for the 10.75x68 duplicating the 404J and having out of spec pressures and cartridge lengths are only good for the target and probably not good for the gun.

Why bother attempting to achieve that, just use the 10.75x68 as it is for fun and get a 404J for even greater fun, and for DG.

Eagle27