Ndnchf- thanks for the link to the info from Kort. I'd seen that before and was searching for it--the site has the best explanation/illustrations I've seen of case movement and primer movement. I got to thinking about this last night in relation to the idea that case head thrust might be a problem, but that didn't make sense . . . .
The firing pin hits the primer, driving the case forward and starting the ignition cycle. The primer is moved back out of the pocket by the force of the primer blast, then stopped by the breech face (the amount of movement is limited to the amount of headspace). The powder ignites. (This all takes about 1 millisecond.) During this time, the case is held in place by friction, and the bullet is starting to move forward. Eventually (about 3-4 milliseconds into the process) the case moves back, re-seating the primer. This movement of the primer is why a revolver will usually lock up when a primed case (not loaded round) is fired: the primer moves back but is not re-seated.
There was at least one design from around 1900 and a few more during the '20s-'30s that used the primer thrust to unlock bolt-action rifles. Garand and several others came up with designs for semi-automatics that used this primer thrust to actuate semi-automatic firearms. In theory, I could see the primer thrust knocking the pin back and moving the hammer, but under normal circumstances I don't think it would be enough to overcome the inertia of the weight of the hammer. I think in event of a catastrophic case failure, the primer thrust is increased by the force of the gas seeking to escape, and that under the right combination of circumstances, this is enough to move the hammer back enough that the breechblock is opened. Wish we could have found the case head to see what it showed.
Maybe. I cleaned and lubed the action on my .43 Spanish rolling block, paying particular attention to making sure the firing pin movement was unimpeded. Then I closed the action and set the rifle at full-cock, and dropped a ~12" piece of 3/8" brass rod down the bore while holding the barrel vertical. The hammer "bounced". How much, and if it was enough to unlock the action, I don't know--it was enough to make me wonder. . . . I don't know why the action on the 44-40 carbine opened, but I think the case failure played a major role. No case failure, no problem: so if I ever get a 44-40, I'll probably stick with neck-sizing the cases and watching the brass like a hawk.
With a rolling block, we've got a couple potential problems that center around the firing pin:
If the firing pin is stuck in the forward position (rust, fouling, peened metal), you can have a situation where closing the breech causes it to fire out of battery. I've done some experiments with this using a deliberately jammed firing pin and primed brass. It looked to me like if it happened with a loaded cartridge it would seriously mess up the thumb that was closing the breech block.
If firing pin protrusion is too much, you can get pierced primers. A pierced primer results in gas blowing back into the shooter's face, which is
disturbing at a minimum. From what I saw (I wasn't the shooter that time!) it looked like the blast out of the touchhole on a flintlock. I don't like to think what it would be like without safety glasses.
Then there are the headspace issues. Early chambers were often what we would think of as "oversize", to allow for fouling, ease of chambering, lousy ammo, etc. Rimmed cases/chambers are usually pretty good on headspace, but in the rimless smokeless cartridges I've seen (7mm, 8mm Lebel) it looked like headspace was established by the bullet jamming into the throat of the chamber rather than the shoulder of the cartridge and chamber.
We complain and worry about this stuff now, but the designers/users of first-generation cartridge guns had other priorities, and didn't have the same knowledge we have today. For instance, I'm not sure they realized the issue of gas handling in event of a pierced primer. Rolling blocks do little if anything to manage gas in even of a pierced primer or ruptured case. But back then they were dealing with different tech to make cartridge cases, still figuring out much of it. As a kid, I knew a guy who lost an eye in WWI due to a pierced primer in a Springfield, something they later addressed with the "Hatcher hole", and eventually, with a hole through the extractor.
Then there is past wear and neglect.. A guy did a great writeup on re-building a rolling block here:
http://www.bpcr.net/forum/viewtopic.php?t=3515