See my posts in cascity..
The point of using Cartridges is to speed things up, and minimize mistakes, by having everything except the cap, in one package. For a flinter, use a bit of the main charge to prime before charging the barrel.
I've tried two types that seem to work, at least for me.
CARTRIDGE, SHOT, ML,12 Ga, Mark I*
This one is the paper tube made by rolling a 3" by 4.5" piece of wrapping paper around a tapered felt dry erase marker that happened to be the right size. I used Gluestick to secure it. It has to be tapered to allow it to drop into the muzzle freely, up to the overpowder card glued into the top.
I squish a conditioned cushion wad to a slightly smaller diameter and push it about 1 1/4" into the wide end of the tube. Measure in shot, then push the overshot card on top of the shot. Clip the 3/8 " of the tube remaining into 6 petals, and glue them down over the card.
While the glue sets, I lube the paper with bore butter, or a BP bullet lube like SPG. I'm still experimenting here, so use your favorite.
Into the narrow end, I measure in the powder. Pinch the paper flat, and fold the sides in to the center and then fold the resulting tail back over the cartridge. They are best carried in some sort of cartridge box like the military ones of the mid-19th Century, with holes drilled in a wooden block to protect them.
To load, take the cartridge and tear off the tail. Traditionally it was torn with your teeth. Pinch the paper to open it a bit, and pour in the powder. Drop the remaining cartridge, paper and all, into the barrel (The one you put the powder in!). It will catch on the overpowder wad. Do the same for the other barrel, if empty, and ram home, tamp, cap & fire.
If you have fired only one barrel, make sure you pick the empty one to reload! My tests show that the unfired charge does NOT move forward, even after about four shots, but check it each time.
CARTRIDGE, BALL, ML, 12 Ga., MARK II
This one was inspired by English Minie-ball Cartridges of the Civil War era. I tried it to solve the problem of a wrapped paper cartridge not fitting the bore snugly enough to be accurate.
Cut the bottom & top off a 12 gauge shell, the one made from tube stock, not the molded one like A-A. Plastic is durable, and endlessly reloadable, but paper shells will be more traditional, and gain more "style points"!
Ram a ball with lubed patch down to the bottom of the tube. Tamp in an overshot card to separate the patched ball from where the powder will reside. Measure in the powder, and seal with another overshot card. I place a bit of string around this card, so I can lift it out to drop the powder in the barrel. I have a 12 gauge wad punch and make overshot wads out of waxed paper milk cartons.
Once the powder is down, place the remainder of the cartridge over the muzzle, and ram the ball down THROUGH the tube. Ram it all the way down, discard the tube (Save to reload), cap and you are ready.
This one would be great for Smoothbore, or Tradegun matches. Most Tradeguns are 20 gauge, and these insructions will work for any standard gauge. For rifles, or odd gauges, you will have to locate, or make, tubing of the correct size. I measured a bit of copper plumbing, and it came to .54! I'll have to try it for my .54 Mortimer flinter.
Keep yer powder dry!