...and why all the mystery about 1870 ballistics?
Here is a whole company founded on hype, the hype that ancient Japanese Samurai were the first "special forces" snipers. Other than my agreement that any military art in ancient Japan was taken very seriously and developed to a high level of skill and art, I'd have to demur.
By all evidence, the first time the Japanese even saw modern European long arms was in the mid to late 1600's, and they were well on the way to being the dominant weapon on the battlefield by then in Europe, whereas they took longer to become accepted in feudal Japan, and were only accepted at all because they were a force-multiplier for area-denial. Prior to reading about Teppo Jutsu, I'd never heard of any Japanese deployment of firearms as anything BUT area-denial weapons, and I'm married into a Japanese family where this sort of discussion has taken place, my wife being only 3 generations removed from the last of the Samurai.
The samurai aspects seem superfluous over at Teppo Jutsu though, and are there to add mystique to their niche marketing of heavy-bullet wildcat catridges/weapon systems in the low supersonic and transonic speed range.
They build an AR which will shoot .458 SOCOM (that "special forces" mystique again!) and something newer called .338 Spectre. OK, I suppose that there is a small need for supressed, low-velocity bullets which can kill at short to medium ranges, but aren't we re-inventing the wheel here?
Looking at the hottest .338 Spectre out there, we find a 180-grain bullet making 1800 fps for a M/E of 1300 #/ft. The round get weaker from there, and to make a subsonic round puts one in the class of, say, a .44 Special. Back at the high end, if you load about 30 grains of Winchester 748 into a .32 Special hull, you'll get about the same performance, out of a gun which might be almost 100 years old.
Let's get modern, though. How about the 50 Beowulf or 50 Alaskan? Those are modern wildcats, and you can load them down to get as solid a hit sub-sonic as you can get out of anything, or load them up and be well beyond that punch a .338 spectre will give you.
Suppressed. Well, OK, suppressed. That's an advantage, most of the time, but REAL noise suppression limits you to subsonic ammo, which limits your range and ability to penetrate cover. It's a trade-off, being quiet, and in this writer's not-so-humble opinion, it's NOT an adequate tradeoff for most war scenarios. You WANT that penetrating power, that long, flat trajectory. You'll give up quiet to get it.
Now, if you're an assassin, and your life depends on getting in stealthy, killing stealthy, and getting out stealthy, by all means have a quiet gun. Maybe what you REALLY need, though, is a cross-bow or even long-bow. THAT'S what the Samurai really used when they wanted to have aimed fire, long-bows.
I can't shoot a bow and arrow. If I decided I wanted to be a stealthy assassin, I would simply have a Marlin 44 Magnum threaded to take a can, and I would shoot subsonic 44 bullets at whatever I wanted to kill quietly. Little fuss and bother that way, and if I can do it with one round, I can take the hull with me, too.
Oh, and by the time that need for stealth arises, all those Teppo Jutsu $3-4K carbines will have been confiscated before they are needed in anger, but my Marlin "deer rifle" will still be flying below the radar.