Spanish Mauser

You can convert 8mm, 270 and 30-06 to 7mm.

a dramatically overlooked cartridge is the 6mm remington. anything a .243 can do a 6mm remington can do better. also there would be literally no tweaking of your magazine or feed lips needed since the 6mm rem is litterally a 6x57mm (necked down 7mm mauser) just be sane with pressures and it would be fine (just like you would need to do with .243) also there's a fun sounding european case, the 6.5x57 but it would probably be easier to go with a 6.5x55 swede just all the x57 cases would literally be only a barrel swap, others might possible need magazine/feed lip tweaking like i said ealier.

edit: Bob, it's a scrubbed rifle like those spanish 95/16's were so it just has a simple serial number and no crest.

double edit: this one's a 93 since it has the square bottom bolt and no safety lug

What are the standard pressures for 6mm Rem? What type of velocity could I expect out of this cartridge loaded for this small ring receiver?

Does your hornady book have pressures? I can find load data but not a lot of pressures

6mm Remington works at over 60K PSI. The small ring mauser belongs in the mid 40K CUP. There is no exact conversion between PSI and CUP ( CUP units being roughly lower in number for the same pressure by 15 to 20% ) but that's still putting the 6mm Rem over the small ring mauser pressure limit.

Then throw in that this actions are 100 to 120 years old and I don't think that's an appropriate chambering.

If you want? I have the reamer and gauges for 6.5x55. Only used 3 times. IMHO don't push the pressure. I rebarreled a Spanish m1916 and the receiver steel and bolt housing steel are so soft I'm afraid to shoot it.

then it looks like 6.5x55 might be the better choice