Wrote this some time back in regards to about the same question, contains some of my others loads also.
SIAMESE MAUSER
450-400-70
The year was 1972 and having just read Pondoro and Use Enough Gun each for the umpteenth time I was determined to have a large bore rifle of sufficient caliber. However, the budget of a young police officer with a family left much to be desired. I then saw an ad in Shotgun News that offered promise. As I had some gun repair experience (military) and tools I decided to build my own.
I purchased the surplus Siamese Mauser rifle for $19.95 along with a Star (later became E.R. Shaw, I believe) barrel that was pre-threaded and chambered in 45-70 for $29.95. The surplus rifle was covered with Cosmoline and when cleaned the action was free of any pitting or corrosion. I quickly stripped it down to the bare receiver and then installed the barrel. The bolt handle was forged, a ramp front sight added along with a Lyman receiver sight and it was bedded with Micro-Bed into the military stock. The stock was semi-sporterized meaning nothing more than the front was cut off and the handguard omitted. The stock appeared to be made of a lessor quality of "also ran" orange crate grade wood. I did not have a lathe at the time so instead of opening the bolt face to fit the 45-70 I chucked the cases in the drill press and with a file turned the rims to fit the bolt face. Thus with 40 cases and a supply of Lyman 457483 400 gr gas check cast bullets I was ready to load. Question was what load?
Some research revealed that 46 gr of H4895 was a recommended max load for the 1886 Winchester or 1895 Marlin. Seemed like the place to start and indeed it proved to be just a starting load. I progressed up to 60 gr of H4895 with that bullet. (have since gone to 62 gr) I ran out of case capacity with no sign of excessive pressure. The velocity was 2050 fps (all stated velocities were chronographed with Oehler chronographs). Recoil with the military stock steel butt plate was brutal! About a year or so later Fajen or Biship offered a completely finished and checkered, 95% inletted, stock for about $50.00. I got one of these and a Packmeyer Triple Magnum recoil pad to go on it. The action was inletted and again bedded with Micro-Bed. Several styles of express style rear sights were tried and the action was drilled and tapped with a 2 ½X Weaver scope being used for a while with excellent results. Thus I used the rifle twenty years for hunting rogue ground squirrels to Rocky Mountain Elk. It wasnt until about 10 years ago I got tired of touching up the cold blue finish every year and had it parkerized shortened the barrel to 24 at the same time. The throat was also reamed out to allow the Barnes 400 gr SP to be seated to magazine length. The bolt face was opened to allow use of unaltered cases. The rails required no modification.
For sights I gravitated back to the Lyman receiver sight and a post front. This provided quick re-zeroing for the different loads and had proven to be the best combination (when my eyes were young) in the hunting situations I used the rifle for. However, recently I found a Redfield base with the flip up auxiliary aperture sight. It fit the hole spacing so I have removed the receiver sight and now have a 2.5X Leupold compact with thick duplex reticle on it. I also corrected a bedding problem and the action is now full length bedded with Steel-Bed. The flip rear sight with the blade front provide quick back up if the scope goes south.
The following listed loads have proven safe in my rifle giving no signs whatsoever of excessive pressure. However, they are for the most part over any listed max loads you will find in any reloading manuals. DO NOT assume they are safe to use. Drop back to published data and work up as I have done. Remember, my rifle is throated longer than standard chambers, thus I refer to this rifle as chambered for the 450-400-70 and have so marked the barrel.
All loads listed are loaded with RCBS dies. A Lee factory crimp die is used to crimp the case into the Barnes 400 gr SP. Winchester cases and Winchester WLR primers are used throughout.
Javelina lube is used with cast bullets (except commercial cast).
LOADS FOR 450-400-70 ONLY!!!
Barnes 400 gr SP w/ 64 gr ReLoader 7. Velocity / 2305 fps
Lee 458-510-FN (GC) cast hard w/60 gr H4895. Velocity / 1947 fps
Speer 400 gr FNSP w62 gr ReLoader 7. Velocity / 2216 fps
Lyman 457483 (GC) 385 gr cast hard
w/45 gr H4895. Velocity / 1495 fps
w/55 gr H4895. Velocity / 1783 fps
w/60 gr H4895. Velocity / 2050 fps
Hornady 300 gr HP w/54 gr H4198. Velocity / 2190 fps
Baker City 300 gr FP w/9.5 gr Bullseye. Velocity / 1052 fps
Rapine 460250 275 gr cast with scrap range lead w/8 gr Bullseye. Velocity / 1052 fps*
*Safe in all 45-70s. This is my favorite load. No recoil to speak of and little noise make this more fun than a person should be allowed. I shoot thousands of these not only in this rifle but in my 3 trapdoors also. The bullets are sized to .4615. They will group into 2 at 100 yards and carry amazingly well to 500 or more yards. Thank God for a Dillon 550B.
Again let me say that this rifle is a 450-400-70. If you have just a 45-70 then treat it as such. However, if you have a Siamese Mauser then be bold but be safe.
Larry Gibson
Good luck, good shooting and good hunting
