Has the buttstock ever been off & the action cleaned & lubricated? If so, then better chance that the firing pins are dragging. If it has never had the buttstock removed, then now is the time to do so. The NRA published a series of books on firearms assembly of most hunting guns. I believe a version of your gun is covered in either volume 1 or 2, but not sure which.
Irrespective, first thing I would look for is any deformation showing on the fired primers of the cartridges you are using. They will clearly bear drag marks, if one or both are the culprit. No marks would indicate the need for a good strip & clean.
w/drag marks:
If you are using reloads, some primers have harder cup material than others, so you might be able to avoid the issue by using a published appropriate recipe that calls for a dif primer.
Same may be said of different factory loaded ammunition; some brands will have harder primer cups than others.
OTOH, you may wish to look closely at the firing pin holes and also the ends of the firing pins to see if the holes are worn or the firing pin ends are eroded or broken. Using a tough material [hardwood or horn] held against the standing breech you can fire both bbls and then check the projection of the firing pins. They should project 1.5mm or .060" maximum through the standing breech. If it is more than that they need to be stoned back to that or a bit less, keeping them hemispherical rather than sharp. Less than 1mm or .040" is considered too little for reliable ignition, so go easy & don't over doit. Too much firing pin projection can cause dragging i.e, hard opening.
Hope something here helps you out. All easy enough to check. Not a big deal to a qualified gunsmith, so shouldn't be overly expensive to get repaired if you do not wish to tackle it yourself.
kind regards, tw
