The original 45-70 used a 405 gr bullet and a 22" twist.
It was soon decided that this was insufficient for infantry use and the blunt 500 gr (now Lyman 457125) was adopted but I don't have the exact date handy. It is about as long as can be shot in a 22" twist. So far as I know the issue rifles and carbines are all 22". HOWEVER they did make some match rifles with longer case cartridges 2.4" I think (not digging for it) that IIRC had 18 or 20" twists using 80 gr of BP. But these were not general issue.
The 22" stabilized the 500 gr Gov't RN well enough AND it allows it grain to arrive point first at maximum range. They found during the beach testing that even at terminal distances the 500 gr would penetrate trenches with 1" plank covering and kill troops below. The angle was very steep close to vertical. They found something like 3" plus penetration on pine at that distance. But they were POINT ON. If over stabilized the bullets tend to arrive with the nose high and can even tumble at longer ranges due to the air flow over the bullet striking UNDER the nose. This causes precession and the bullet nose tends to oscillate around the flight path. This causes some modern HV loads to strike the target while tumbling at 1000 yards.
It is also why some modern tanks use smooth bore guns. It is important that the solid shot strike point on at longer ranges.
So its possible that the Army did not WANT the 500 gr any more stable than it was.
The action was repeatedly upgraded with the 1884 being the best issue action. Some collectors do not consider the 73 or 78 models safe.
This is not a smokeless powder action. There is loading data out there that is based on the SAAMI pressure levels for HV SMOKELESS LOADS of the early 20th c. These generate far to much pressure for the TD. The Gov't considered it to be able to contain 80 gr of BP MAX from my reading.
Like all BP rifles of the 19th c bore sizes vary a lot. The use of 20:1 or 40:1 alloy or even lead will over come this with BP as the propellant.
Care must be taken with these actions and barrels with BP substitutes are used. Excessive compression of Pyrodex will produce excessive pressure and I have been told of a TD having its barrel burst by this practice. T7 IMO is too hot for the action.
Reproduction TDs have been known to blow open with BP loads. I was personally told of 2 such occurrences some years back when the Italian distributors started importing TDs. One shooter said the case cut a groove in his hair. I was told second hand that an H&R had done the same thing.
How can this be? The TD action was designed around a very poor cartridge case that had no springback. The folded copper case the Bureau of Ordnance liked so well and killed troops during the Indian Wars. If the TD could not be opened under pressure they would have been jammed shut routinely by the copper cases. So the cam lock is a fine line between blowing open and being locked shut with the issue ammo of the time. Apparently some of the reproductions did not get the cam and its recess just right.
So, shoot BP only (light loads of smokeless can be really dangerous) use fairly soft bullets (the originals were 1:16) and I would recommend only shooting the 1880s models since there has to be a reason they repeatedly beefed up the action.
Under no circumstances should HV 1600-2000 fps loads be used in any available version of this action no matter when it was made, no matter who's reloading manual its in. 28000 psi is the SAAMI standard for the TD and these loads were intended for 1886 Win, 1895 Marlin and other strong actions. The TD far predates SAAMI or any of its standards. The failures years ago in weak 19th c guns by Ballard, Soringfield Armory, Winchester and others with the HV smokeless loads of the early 20th c was the reason all these loads were removed from the market by the 1950s and 45-70, 44-40, 38-40, 32-20 etc were all loaded to BP pressure levels rather than the really snotty loads that were developed for the various smokeless capable actions of the late 19th c. What is safe in an 1892 Winchester may be fatal in a 1873 model using the same cartridge for example.

Note. Some know that the 30-40 cartridge load development was done using TD actions. This is true. HOWEVER, they were redesigned, stronger actions than the issue actions. But some not knowing this, assume the TD is much stronger than it is based on reading that "TD actions" were used with the 40000 psi + 30-40.

Dan