"imho its allways possible to fire the original 8x56 MS in this rifle"

WHOA THERE! I would beg to disagree with this statement. The location of the shoulder and the angle of the shoulder provide for an excessive headspace issue when firing 8x56MS in an 8x57JS or I chamber. The shoulder starts at 1.819" on the 57 case and at 1.80 on the MS. The MS also has a steeper taper 22 degrees vs. 19+ of the 57mm case. Given the chance of maximum chamber and minimum case deminsions, I believe you are asking for trouble. Maybe not disastrous, but surely problemmatic. Secondly, the 8x56 uses a .323 dia. bullet, while the I uses a .318 diameter. While that would relieve pressures, it would do nothing benifical for accuracy and limit bullet selection.

When I had my 1908 8x56MS half stock carbine, I used Remington 8x57 Mauser (JS) cases that I resized in RCBS 8x56 MS dies and trimmed to length. Worked great and lasted forever. I did not own an 8x57 of any type at the time so wasn't worried about mixing ammo. The MS is long gone and I recently picked up a nice O.G. Scherell pre 1913 sporter in 8x57I. First thing I did was go looking for the left over ammo for the MS so I could get rid of it and not run the risk of a oversize bullet and excess headspace combination.

I would sugest the OP slug his bore and he may find it suitable for .323 bullets and have a larger selection. That with proper 8x57 sized brass (for proper bulllet diameter) will give him a woods rifle that is a joy to carry, shoot and let others admire. Mine accounted for more than one large Maine Whitetail when I had it and if it wasn't for me being left handed and that butter knife bolt being a bear to operate, it would still be in my safe.

Just my .02 and HO. Far better to do it right (and no more trouble) than risk injury or gun damage.
Thaine