Actually, no. On the first part, yes breech plugs let loose, their threads are actually one of the weakest parts of the pressure vessel. I can reference you to even a medical journal article on a breech plug killing a shooter by ending up in his brain, and multiple posts from people more fortunate.
Also, pressure is not just pressure. The way that pressure develops is critical as well. Ever noticed how certain machine parts specify certain bolt grades? That's because pure strength is not everything, but the way that bolt is used matters too. Sometimes a lower grade bolt will be specified and last longer than a high grade in a certain application.
Smokeless powder has a drastically different pressure curve for most powders than black powder, the pressure peak is later, and sharper. We are not firearms engineers, at least I don't think so, and using a propellant not designed for an application is, to use the same term again, asinine; whether you can get away with it 'most of the time' or not. Manufacturers say no. I have no reason to pull out a tire and string and try it. Why on earth would I? If I want to shoot smokeless, I shoot a modern gun. If I want to shoot black powder, I shoot a black powder muzzle loader or cartridge gun. If I want to shoot a muzzle loader with smokeless, there's a gun for that too, the Savage. Use the right combination of gun, pwder, and components and you'll never be dissappointed, or maim/kill yourself or others, or at best ruin your gun.
There is just no reason.
Finally, .375 H&H was never designed for black powder firearms even. You are very clearly making assumptions. The .375 H&H was introduced in 1912, purely to smokeless powders (which include cordite), and nitro proofed actions. The Mauser M98 Magnum was its first chassis and the gun in which it was introduced by Holland & Holland. Not until the 1930's was it introduced in a new gun, the Winchester Model 70, also as I'm sure you know is a strong nitro proofed modern firearm and a far cry from a black powder firearm. So no, it was never a black powder round as you first stated, and never was designed for black powder actions as you've stated last. You're plain wrong and are spreading false info, based on your own incorrect personal assumptions. I don't like to try and be an internet tough guy, or speak ill of people, but your post might likely be one of the first I've written black with an explicit bold and red text warning. It's just plain dangerous, wrong, and it shines through with comments stated as facts such as your wildly off base .375 assumptions.
Contrary to what you may think, I do know what I'm talking about. Pressure is pressure. If a charge of BP generates a certain maximum pressure than producing that same pressure CURVE by another means (compressed air even) is going to have the same net effect.
Your breach plug will handle the pressure of a BP charge just fine. The savage breach plug will handle more. The savage breach plug will allow you to shoot hotter loads. BP generates a much spikier pressure curve than most smokeless powders with max pressure occuring very near ignition. Smokeless pressure reaches maximum much later. You're more likely to split the barrel than shoot a breach plug through your neck. How come you never hear about people putting a breach plug through their neck when they overcharge a muzzleloader or plug the barrel? Because it's not the weak point.
Go get a transducer and a spare tire and try it out. Just do it safely. It's not because people played around outside manufacturer's recommendations that the firearms industry is lawyered up - it's because people didn't do it safely. You're not going to get much higher than 15ksi out of a shotshell primer before you start popping the primer anyways.
Would you shoot any handload that wasn't listed in a reloading manual based on assumptions without doing it _safely_?
Also, for the record, I never said the 375 H&H was a black powder round. I said the round was designed for a BP rifle. I was mistaken. I was under the impression that it had followed the same path as the 303 british did with the Lee Metford. I did state the 375H&H was a cordite charged round - which has a similar pressure curve to BP - just not by weight or propellent. Cordite is very high in nitroglycerine. More like a duck and a giant goose using your above analogy...
Be safe.
