between the left side wall and the receiver ring. This means that the left receiver wall is much thicker in the Siamese receiver, and the reason that it is so made is because of the breech cover guide rails. The average M98 Mauser left side wall is about .155” thick, while the Siamese Mauser is about .200” thick.
5. Bolt face. The recessed bolt head will accept a cartridge rim no larger than about .564”. There are no lips extending forward on the left side of the rim recess as in the regular M98 Mauser action. The rim recess is cut away at the bottom to allow the cartridge rim to slip under the extractor hook on feeding a cartridge from the chamber, which prevents double loading.
6. Extractor hook. It is narrower than the extractor, being only about .300” wide.
7. Tangs. Separate upper and lower tang extensions on the 8mm Siamese action extend rearward about 3” and are inletted into the top and bottom of the grip of the stock. They are milled at the front end to fit closely against the rounded end of the integral receiver tang and trigger guard, and are held in place by the rear guard screw. The tangs, connected at the rear by a screw, are almost exactly the same as those used on the Type 38 Japanese Arisaka 6.5mm rifles and carbines. They are used to strengthen the grip area of the stock.
8. Magazine. The combined trigger guard/magazine is of one-piece milled steel construction like the regular M98 Mauser. The front and rear ends of the magazine box, however, are slanted forward at a sharp angle. This slanted magazine box permits easy loading of the larger rimmed 8mm Siamese cartridge, the slope preventing their rims from catching on each other. Each cartridge pushed into the magazine moves to the rear so that the next cartridge inserted over it will have its rim ahead of the cartridge rim below. To gain adequate magazine capacity, the rear of the magazine box was made about ¼” deeper than the regular M98 8mm Mauser. The floorplate of the Siamese action is not hinged, but is quickly detachable via a latch built into the forward part of the trigger guard bow. Due to the slanted magazine box, the trigger guard bow is positioned back farther than the regular M98 and for this reason the trigger shank is bent back as mentioned earlier.
Bolt head of the Siamese action, showing: (A) twin gas-vent holes; (B) extractor; (C) right (bottom) locking lug; (D) undercut cartridge head recess; (E) ejector slot, and (F) slotted left (top) locking lug. The bolt also has the regular M98 Mauser short guide rib and the rear safety locking lug (not shown).
Incidentally, the front and rear guard screws of the Siamese action have the same thread pitch as the regular M98 Mauser guard screws, but the Siamese screws are slightly larger in diameter. However, regular M98 Mauser guard screws can be used in the Siamese action, but not conversely.
The 8mm Siamese Cartridge
Generally referred to as the 8x52Rmm Siamese, this is a rimmed, bottlenecked cartridge normally loaded with a 181-grain pointed, jacketed bullet. Not much is known about it—when it was adopted or who developed it. It’s probably merely a ballistically improved version of the older 8x50R Siamese Type 45 cartridge; a little longer case, a pointed bullet and more powder.
Fred A. Datig in his book Cartridges For Collectors designated this cartridge as the 8x52R Siamese Type 66—so does George C. Nonte in his book Home Guide To Cartridge Conversions. I don’t know what the “Type 66” means. Regardless, if you have the Siamese Mauser rifle and want to shoot it, there is very little chance that you will ever find any factory-loaded ammunition for it. If you are a handloader, Nonte suggests the following: make cases from new 45-70 brass, trim to 2.04” and turn the rim to .560” diameter. Full-length resize in a 33 WFC die, size down the neck and push the shoulder back in an 8mm die until the case will chamber, then fire-form. Use .323” bullets.
Comments
The 8mm Siamese Mauser action I received was in very good condition. Very well made and finished, it was as smooth in operation as any military M98 Mauser I’ve ever handled. I have no way of knowing the kind of steel used in making the receiver and bolt, nor do I know what heat treatment was given these parts. If I might guess, I’d say that if the arsenal in Japan that made the Siamese Mauser actions also made Arisaka actions, the same steel and heat treatment were used for both. I know that a file test for hardness is none too reliable, but it can be used to compare the hardness of such parts as rifle bolt heads. To do some comparing I took a file to the locking lugs of a couple of M98 German Mauser bolts, but could not get a good bite. This is normal, for most M98 bolts are very hard. Then I tried the file on the Siamese bolt and I could cut both locking lugs and the front edge of the bolt. Next, I got out two each of the Japanese Type 38 and 99 Arisaka bolts and, with the same filing effort, cut all four of them. Because the M98 Mauser bolt has smaller locking lugs than the Arisaka it is probably a good idea that they are made very hard. I don’t think they have to be made as hard as they are for strength alone so I see no reason why the softer bolt in the Siamese action wouldn’t be just as strong. However, to be on the safe side, the Siamese Mauser action should be limited to cartridges that do not develop much over 45,000 psi breech pressure.
The 8mm Siamese cartridge (8x52R) has a rim diameter of .561” and is about 2.925 ” overall. You’ll recall that the ad I read called this action suitable for the 45-70, 444 Marlin, et al. I checked a number of cartridges in this action, and here’s what I found.
45-70. The 45-70 rim is .608” in diameter, too big for the .565” diameter recess in the bolt face. The front of the magazine well (cartridge guide lips) is also slightly too narrow to pass the 45-70 rim. The unaltered magazine will accept and hold 45-70 cartridges, but the bottom or last cartridge is not tipped high enough in front for it to feed correctly. To use 45-70 cartridges, the bolt face recess must be opened up to about .610” and the front of the magazine well widened—not difficult to do. The latter job will most likely end the feed problem of the last cartridge. With these modifications this is a good action for the 45-70 cartridge. Or do as the late George Nonte suggested; turn the rims down to .560 diameter.
444 Marlin I don’t think the use of this action is going to be too successful with the 444 Marlin cartridge. The 444 rim diameter is only about .514”, and the 8mm Siamese bolt face recess is too large for the too short extractor to hold this cartridge in place for proper ejection. Also, the magazine will not retain the first cartridge inserted into it; similarly, if the magazine is loaded with more than one cartridge, the last cartridge will pop out after the other cartridges have been fed. This last problem can be corrected by rebuilding the follower, but I know of no practicable way to reduce the bolt face recess. Therefore, if you want to use this action for the 444 Marlin I’d suggest you solve the bolt face recess and magazine problem before going to the trouble and expense of fitting a 444 barrel to the action.
303 British The 303 British cartridge is normally loaded to an overall length of 3.075” and it will just enter the 8mm Siamese magazine, with little length-room to spare. It has a rim diameter of .540”, and it fits the bolt face recess OK, although for better ejection of the fired cases the extractor hook should be a trifle longer. As with the 444 Marlin cartridge, the 303 British case rim is a bit too small for the rear width of the magazine well, and in order for the magazine to retain the first cartridge inserted into the magazine, or the last cartridge after the others have been fed, the right side of the follower ridge (left side of the follower) should be built up slightly. This can be done by sweating (soft soldering) a thin piece of sheet steel onto it. The 303 British cartridge does not fit this action perfectly, but it comes very close, and with the modifications to the extractor hook and follower, the Siamese Mauser action should prove to be ideal for this cartridge.
Left side of the Siamese action with the breech cover, but minus the tangs.
