gzeck,
The 10.1 is the bore diameter.The nominal caliber is likely to be somewhere around 10.3-10.75mm. The fact that a 45-70 case was used to make snapcaps indicates that the case is in the 43 Mauser(11.15x60R)family.This is reinforced by the impression that the rim recess seems to be deep enough to accept the so called "A" base or"MB".When 43 Mauser cases were not avaliable,45-70 or 45-90 cases were used by turning the rim and necking the case to headspace on the shoulder,instead of the rim.To determine the nominal caliber, case length,and bullet diameter(groove diameter of the barrel)should be determined. The bullet in the snapcap may or may not be che correct dia.This should be confirmed by "slugging" the bore.Case length should be determined by a chamber cast. The snapcap may or may not be the correct length.If it has been trimmed shorter than 45-70, it may be correct. If it is 45-70 length,then the length should be confirmed. There is "no doubt in my military mind" that useable ammo can be made for this rifle.
Mike