What you have to do remove the stock and that will allow access to the firing pins which are actually bushed but screwed into the breech from inside ( just the opposite way from most SxS guns where you unscrew the bushes from the breech face). In Merkels these bushes do not show at the breech face so you wouldn't know about them until you remove the stock. Sometimes they get unscrewed a little with the the result you describe. You just have to tighten 'em in, don't use locktite, the next person will curse you. The trick of course is getting off the stock.
In addition to removing the tang screw/screws, you also have to drop out the trigger plate and trigger guard. This is finicky work requiring perfectly ground screw drivers and experience so as not to bugger the screws. About 90% of fine gunsmithing consists of NOT buggering up the screws.
nial
