Every one in awhile I have trouble opening up the action on my 40-70 SS Hepburn after firing a round.  I have decided now that this is caused by the primer backing out enough to make it hard to open the action.  The fired case falls out of the chamber when the action does open.  I am using Hornady 405 Brass in this modern barrel and it looks to be flush with the chamber when seated.  I have had this with both smokeless and blackpowder loads.  The primers all look like they have been hit with a sledge hammer and center punch as the mainspring is stout to say the least.  I am not familiar with how the rebounding hammer works but I am wondering if that has something to do with my problem.  I fired some low pressure smokeless loads today and the problem seemed worse but that might have been a coincidence.  The primer pockets are snug.  Help Mr Wizzard!     Tom

Westener, do you fire twice just when the action won't open?  I assume that in doing so it pushes the primer back in so everything works as it should. I will inspect the shape of the firing pin and change to a different brand as well to see what works.  Like I say it is maybe 5% of the time at most.  Tom

De haas explains how to fix this problem in his single shot book, the problem is the rebounding hammer.  Ledball

When it was mentioned that the problem seems to get worse with lower pressure loads, I had the thought that there might be a bit too much headspace for how ever the loads are set up. In this case, the primers are actually backed out and not just firing pin drag? I think the effect can be seen in a revolver, fire a regular round and all is normal, fire the same primed case and the primer locks up the cylinder. Possibly, in this rifle, if the lock up happens less at higher pressure, the web of reloaded brass might show signs of a bit of thinning? I do want to dig out the DeHass book and see what he had to say though.

I  believe his solution is to get rid of the rebounding feature of the hammer...

I'd really like to know what's going on Tom.   
I built a 38-72 Win in a High Wall and the head space is very close now as I had this problem with it.  I thought maybe excessive head space, so I indexed and recut the chamber only giving it .002 over the brass I was using, not the actual gauge.  I still got a "popped" primer occasionally and if I let the hammer fall a second time the block falls easy.  I experimented with several loads, black, duplex, and smokeless.  Someone said something about trying different brand primers but I don't recall the details and never tried it.
When I switched from Hornady 9.3 brass to Nosler 9.3 brass all my issues went away...   
The 40-70SS I'm building right now is being put together with the intent of using Hornady 405 brass.   I'm not convinced that the rebounding hammer is the only culprit here.  If you haven't tried a slight crimp on the bullet, you may give that a try.
I thought I had an interesting article about the series of events that happen in a large volume case, specifically long skinny cases when low operating pressures are prevalent.  Maybe it was a dream or passing thought in this pea-brain?  Someone here have something on this - chime and help me keep my sanity...?
Greg

Often thought about that.
As I mentioned I know I read a study about this now and there was quite some discussion.  This was what pushed me to purchase a pressure trace system, it's still in the box on the back of my desk so that tells you how far I got with it...

Cases do not show any drag marks.  I took the block out and the firing pin spring appears strong to me but nothing to gauge it by.  The firing pin tip is rounded.  Now the kicker.  I measured the rim recess in the chamber and it is 0.073.  By applying tape to the case as John suggested I got 6 layers and it measured 0.012.  The face of the block looks perfect.  I will keep better track of when it occurs but in the meantime I will cock and fire again.  If I go after Lions I will hunt with my 7x57 like Karamojo Bell!     Tom