It doesn’t. In fact, there are slight indentations in that '06 casing just from the light pressure I applied, and the bolt didn’t come anywhere near home.
Sure you could mill it down, that’s how reloaders make cartridges for rifles that have ammo types that aren’t easy to come by, like the Steyr 1895, but machine shops aren’t readily available in the field, and who wants to pull the slugs and dump the powder on a handful of cartridges, then reload them all after all that machine work anyway, when they could just pick up a weapon that was designed for that cartridge and get back to fighting?
Surplus ammo is still readily available for most of the popular vintage, surplus rifles, and much of it is actually in good working order, though it’s nearly all corrosive. There are plenty of sources for original 8mm ammo. I don’t have any, and don’t care enough to want to get any, as the new ones work just fine and aren’t corrosive, but it’s out there if you really want it.
I don’t want to get into a, eh-hem, “exacto knife” measuring contest about it, but barrels are machined to specific dimensions and there’s really not that much “slop” to allow for minor differences in casing shape and design. A new casing isn’t going to differ that much from an old one, since it has to physically fit in the chamber. The period 8mm cases I’ve seen are the same as the new casings. You can’t get a wider rim to work without modifying the bolt. It’s machined for specific dimensions, as is the extractor.
A smaller bullet will work, but if it’s too small, the least you’ll have to worry about is that it’ll be grossly inaccurate, and will likely damage your barrel beyond repair. It could even be dangerous. Damage to the casing, while trying to force the proverbial “square peg” into a “round hole” can be dangerous too. Even if you made it work, and managed to pop off a shot without the rifle exploding in your face, good luck getting the casing out. Good way to make rifle worthless when you need it most.
Anyway, I’m still hitting a wall with the Mauser 91. My sources say the Argentine model, which was also exported to several other South American and European countries, is the only Mauser bearing the 1891 title. It was made in Germany and exported to Argentina, but was not issued officially to any German forces. The only German Mauser between the Model 79 and the classic G98 of WWI fame is the 71/84, which was an update to the Model 71.
The Argentine 1891 (http://www.surplusrifle.com/argentine1891/index.asp) bolt featured the very infantile stages of what would become the classic Mauser bolt. You are correct about the caliber though. It was 7.65mm. But the rifle differs in many ways from the classic Mauser in shape, the most obvious being the Mosin-Nagant style magazine sticking out of the bottom, in front of the trigger guard.
If that is indeed your rifle, then I did figure out the bayonet situation, and in answer to your question as to whether or not your new bayonet would fit, the answer is no. The bayonet for the 1891 was similar in basic design, but the rail is on the bottom of the nose cap, rather than in front of it, as on the G98 and 98k, so the ring on the hilt, that fits around the barrel has to be farther away from the blade than on a bayonet for a later Mauser. That’s assuming what you have is indeed the same rifle I’m talking about. Yours says Berlin on it, so it was almost certainly made at the Deutsche Waffen Und Munitionsfabriken arsenal, and it probably says so somewhere on the reciever. In fact, I’m looking at the side rail of an Argentine 1891 in a picture right now, and it says “Mauser Modelo Argentino 1891, Deutsche Waffen-Und Munitionsfabriken, Berlin”.
What you want to do is check out the top of the reciever and see if there’s a national crest up there. That will tell you who it was made for. Most likely, it’s the Argentine national crest, which has an oval with two hands shaking and a torch (which looks an awful lot like a santaclaus hat on a stick), and horizontal lines from the middle to the top. The oval is surrounded by a wreath and there’s a sun rising at the top of the whole motif. There were other crests too, that denoted rifles issued to different branches of the Argentine military, as well as miltary academies. It’s also possible that it was made for another country though, so the crest is what matters for identification. If it says “Deutsche Waffen-Und Munitionsfabriken, Berlin” and a year, instead of a crest, then it was made for the German army.
My Czech VZ24 Mauser was made by BRNO and bears the BRNO markings on the side rail, with the rest of the writing in Czech (but the crest, which has been largely obliterated by a grinder) is the Romanian crest of King Carol, which means that the rifle was made for export to Romania for use in their army. The only non-matching serial numbers are on the bolt handle and the floor plate of the magazine.
That your rifle was taken as a trophy in the ETO during WWII means that either it was never exported, or had been sold back, possibly as part of an exchange for a shipment of newer weapons, and was pressed into service as part of the last ditch effort in the waning days of the Reich.
