Muzzleloaded for about 20 years and it IS a cap box and was used on both shotguns and rifles, also some pistols although these were usually more round and went into the butt like a buttcap. Mounted as William Lowe indicated. Put caps in it, when needed turn the gun with the trigger guard up, open the cover and tip out what you need. They really work quite well.
Some folks do/did use them to hold a jag and in smaller calibers like a 36 Cal. Squirrel gun they could be used to hold precut patches, providing the hole it large enough.
It would work very well for what you are considering using it for although as someone pointed out a nice inline capper is easier to use....
Have a Great Day All...
WBLDon
