Hal, I can’t say much from personal experience, as I don’t have one of those guns fitted with ‘movable chambers.’ On pages 11 and 12 of this thread are Beringer guns equipped to operate as pinfires or with cartridges/removable chambers and percussion caps. I’ve only seen photographs of metal cartridges with an external nipple that could fit in a pinfire gun (page 66 of Macdonald Hasting’s book, English Sporting Guns and Accessories, if you have it). Loading such a cartridge or ‘second breech’ would be like loading a primed hull by hand. I have used brass ‘everlast’ pinfire cases, and inserting a percussion cap and pin in one of these is probably more fiddly than using a case with an external nipple.
The type of chamber insert depicted in the Westley Richards above would seem to offer the choice between using a cap, loose powder and shot, and a pinfire cartridge. I can understand the reasoning for such a choice at the beginning of the pinfire era, as one would not be sure if pinfire cartridges would be available on, say, an extended hunting trip to distant lands. However, the pin holes in the barrels are much larger than with a typical pinfire and would not necessarily keep the pins perfectly upright, an immediate cause for a misfire. Again, however, without examining such a gun first-hand, or reading contemporary accounts of their use, I’m just speculating.
