Frank:
If your Siamese Mauser is like mine, you will have a lot of extra cartridge length available in the magazine. This will let you shoot 45-70 rounds that have a COL of greater than the standard 2.54 inches - provide you have a bullet shape that will fit the free bore in your chamber. You can also take advantage of Spritzer points.
This brings me to the Barnes Banded Solid I mentioned above. Since it is 330grs of brass it is longer than a standard 350 gr JFP - it is roughly similar in length to a standard lead/jacketed 400gr projectile. Since it has driving bands, if it is seated/crimped in the grove just behind the driving band closest to the point you get similar case capacity as if you are loading a 350gr XXXX. The driving bands give it a slimmer nose profile an allow a round of a little over 3" COL to chamber in a standard 45-70 chamber. I can even chamber one in my Marlins if I drop it in through the ejection port - Warning: It will not eject unless you actually fire the round or pull the bolt out - and don't fire one of my hot loads in a Marlin!!!
You may want to try a different powder than RL-7 if you want more velocity with lower pressures. As posted above I use H4198. The three Mausers I have have 20", 22" and 26" barrels. The 20" runs my loads at 2400, the 22" at 2460 and the 26" at 2710 with zero indications of over pressure. Note that these are pretty compressed loads.
I am currently at 56grs of H4198 in the Win 70 in 450M and I am just at 2400fps. The 450M case has a 5-7% smaller volume due to its belt/webbing.
Based on last years performance on plans game I would not be afraid to use the solids on everything that walks the planet...just be sure of what is on the other side!!
