The key to getting out it, short of disassembling the gun completely, is "wiggling".
There's really no room to put something down the tube and be able to wiggle the cartridge side-to-side, so you're only left with up&down or rotation.
If I were having such a problem- I would go hunting for a piece of tube or pipe that will fit down the cartridge tube to protect the rifle- then either use a small rod of some-sort with a bit of glue on the end (i.e. a steel epoxy, or maybe 'Krazy Glue'), let it dry completely, remove the "tube" you put in and try to pull the caplet out. Of course you would have to use a long 'Q-Tip' type of thing with some solvent or maybe alcohol to clean any lubricants you put in there off the top of the caplet- or no adhesive has any chance of sticking. If that didn't work- I'd probably move on to a long "easy out" type drill bit and a reversible drill- using the tube protector of course- to try and "grab" it a bit and rotate slightly in order to break it free.
You could also try rapping the butt of the rifle on a hard surface (i.e. a concrete floor) to try and knock loose the caplet- but use a towel between the floor and rifle to avoid damaging anything. Also note, by "rapping" I don't mean hammering it to death, just a couple of reasonably solid hits with no "bounce" off the floor.
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