Let us quote the rest of the article [Guns,Aug 1999]:
"When you want to use smokeless powder to duplicate the pressures of the old ammo, there's a lot of space left over.
Bad things have happened to people who didn't recognize the potential for disaster caused by that excess space. One of the myths explains the catastrophes that have befallen .45-70 shooters is that small charges of smokeless powder detonate with explosive force. There's only one problem with that story: it cannot be duplicated in the laboratory.
Powder and reloading authorities tell me that true detonation simply cannot occur with the relatively small quantities of smokeless powder used in any handloads.
Well, if it didn't detonate, what blew up those guns? Let me be the first to say that I don't know if the things we have seen with .45-70s are caused by the theory I'm about to report, but it is certainly plausible. Metallurgical examination of some guns that were destroyed doesn't show evidence of detonation, it shows that the failures were due to high pressure, but powder charges were relatively small. How could that be?
When gunpowder burns it produces gas under pressure to move the bullet. As the bullet moves down the barrel, the volume of the combustion chamber increases. If the volume increases at a rate greater than the rate of gas production then the bullet can slow down or even stop in the barrel.
If the bullet stops, it becomes, in effect, a bore obstruction. To paraphrase Mr. Newton's law: bodies at rest want to stay that way. And they don't care even a little if that behavior happens to make your gun blow up.
If it weren't for the fact that the volume of the combustion chamber (cartridge case and bore) increased to reduce the pressure, a relatively small powder charge could wreck almost any gun, but usually it takes a combination of bad things happening to do it. The .45-70 is a potential candidate because of all that extra powder space."
[Guns Magazine, August, 1999 by Charles E. Petty]
I understant the difference between a propellant and an explosive. Putting a small amount of Unique in a large case doesn't convert it to TNT.
The point here is this; Small amounts of powder in a large case does not act any differently. It can however, cause your gun to blow up.
A small charge of slower powder in a large case can cause your gun to blow-up, for no other reason.