'”The 30-06 is my preferred round and I've read others recommend it but I tried to load fmj rounds and they are too long'’
The 8mm57 is shorter from the head of the case to its shoulder .121” than the 30/06, meaning when a 30/06 case is chambered in the 8mm57 chamber there is .231’ protruding from the chamber. I do not know the condition of your barrel, back when cases were difficult to find for the Mauser the chamber was cut to 8mm06, back to your question regarding what will work without modifications to the magazine, the 8mm06 is the same length as the 30/06.
Rich S, If the rifle is in full military dress consider applying the leaver policy, leaver the way you founder. When I am told a rifle is worth $700.00 +/- I say to the person doing the evaluation to ‘PAY ME’, they never do so I build what I choose to build.
I do not have less than 30 Mouser barrels, most are large shank 98K type barrels, when I replace a Mauser barrel I check the length of the chamber first from the bolt face to the shoulder of the chamber, in the perfect world the difference in length between the chamber and case from the head of the case to its shoulder is .005”. After removing the barrel I check case head protrusion, case head protrusion on the 8mm Mauser barrel is .110” +/- very little, I know none of this makes sense to anyone on this forum but I chamber the barrel first then installed on the action.
I purchased 4 Mausers for $25.00 each, I used one barrel to test all four receivers, with 4 receivers and 4 bolts there was not .001” difference in the length of chamber with one barrel, when checking the receiver the distance from the ‘C’ ring/torque ring/barrel mating surface down to the bolt face is the most important one, I am never surprised when the measurement is .115”. The second measurement is obtained by measuring from the barrel face to the shoulder at the end of the barrel shank. The barrel face must must hit the ‘C’ ring before the front of the receiver ring hits the shoulder at the end of the sank. (easy to do), screw the barrel in until it contacts the ‘C’ ring, then measure the gap between the front receiver ring and shoulder at the end of the barrel shank with a feeler gage. Slack will be removed between the threads and there will be some crush but the face of the barrel can only get so close to the torque ‘C’ ring, then galling begins.
I use the barrel vise in a press, there is no such thing as too tight.