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After a fairly extensive search I cannot find my Lyman Cast Bullet Handbook which has pressures for BP loads. I know that the 45-120 topped out at about 29 to 30K with 120 gr of powder and a heavy bullet.
May have to buy another cast bullet handbook. I do know that Duplex loads exceed Lyman's 18K pressure limit for the TD. A limit that they have maintained for decades. Since Lyman has more experience in this that any company I know I tend to take their data at face value. I have no reason to doubt Accurate either, but at that pressure level they should be making velocities well in excess of BP loads in the 45-70. Duplex loads in 45-70 WILL exceed Lyman's limits for the TD.
70 grains is more than modern cases will hold if Goex is used. 62-65 is about max. With lighter bullets and some compression I have shot a three groove 350 gr and 75gr of Swiss, which is denser than other powders from a 45-70.

But the main reason for not shooting smokeless in archaic breechloaders, even when made of modern steels is the lack of safety features to protect the shooter. Even strong 19h c designs like the Winchester Single Shot (Hi-Wall) have serious deficiencies for high pressure use. Many are prone to spit out firing pins for example. I have read of hiwall hammers being sheared off by this.
Loads that will kill or maim in a modern 1873 Win copy will not even stick the case in a 1892 Win.
If the loader never exceeds about 18000 in the TD with smokeless and takes precautions to make sure the loading density is such that there is not chance of what a friend calls "flash over" then smokeless is fairly safe. But there is still the potential for problems. There is a big name in the shooting/reloading world who is now missing some fingers due to using 24gr of 2400 in a TD. He was told it was a bad idea but had shot the load for decades with no issues. In a couple of years the barrel let go and he lost parts of his hand.
Its just not worth it to me. I stopped using smokeless in vintage design BP arms due to the safety concerns, lack of modern safety features (Neidner developed a blow out proof firing pin for use in Hi Walls converted to HV varmint cartridges like the 219 Zipper) and the fact that the steel in the barre especially is questionable.
Not so long ago a lady I know lost part of here hand from shooting smokeless in a cast action Ballard chambered for 40-65. Even loaded with BP the 40-65 is more than the cast action will stand. But people like to do what they want and ignore warnings that even back in the day Ballard never used this action for cartridges larger than 38 RF. About like a BP 38 Special.
So while there are some 19th c firearms that can be used with smokeless, the 1886 and 92 Win for example and those designed with smokeless in mind are pretty good. Most are not suitable due to lack of what would be basic safety features in the smokeless powder world. We have to remember that Sharps, in the 1870s installed gas check plates, the actually worked, in all rifles chambered from more the 65 or 70 grains of black powder. It was actually safer and still is in this regard than the Rolling Block, the more "modern" Hepburn, the newer Hiwall and the Ballard.
Then if we step the cartridge capacity to 100-120 gr of powder it is much more difficult to safely shoot smokeless, I don't recommend smokeless in any cartridge that used more than 70 gr of BP. Because people blow up guns using almost any smokeless you can name when the loading density gets to low.

Dan

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