I have no idea how many rounds would have to be fired in one bbl to transfer enough heat to the other bbl to produce a "Cook-Off".
This is normally thought of as a problem only with full automatic guns such as machine guns.
Other than that it would seem the hammer had to fall. From your description it seems there was othing to produce a jar other than the firing itself. It is possible the hammers are not quite timed together (They Should be of course) & that you were only opening the gun enough to re-cock the upper bbl without re-setting the lower.
This could be checked out rather easily working with snap caps to see how closely he hammers cock together.
ould agre it is a good idea to get the maker involved. A gun firing while pushing the opening lever is definitely not a good idea.
