We have lots of good answers here. I don't know that I can add much to the fund of knowledge, but will offer some thoughts.
Not all ball pullers or screws are created equally. The screw-type ball pullers from Track of the Wolf are specifically designed for this task. They have sharp points, deep, sharp threads, and a proper taper. The collar keeps it centered. Track has the right tool for the job.
The two-pronged gunworms or "wipers," now commonly called "tow worms," used to be made with the points angled slightly down. Somebody already alluded to this, but it bears repeating. These could be wrapped or entwined with tow for swabbing the bore, but could also be used for pulling wads from a smoothbore, or retrieving a lost patch, but not for pulling a ball. The custom "tow worms" that I have seen recently from a couple of makers/vendors are made with the points flat or horizontal. They are nicely made and work well for wiping with tow, but they don't work for pulling anything.
In olden times, it was standard practice to plug the vent of a flintlock with a feather, to keep it from leaking powder, to keep the vent clear, and to keep moisture from getting into the powder charge through the touch-hole. This is pretty well documented. Several fellows mentioned using a toothpick for this, but I think a feather quill would be more compressible, to make a better seal, and more flexible (less likely to break). I feed birds, and see small feathers in the yard almost daily. Some of these may not be legal, though.
I have had to clear three guns so far this year. All were percussion. One was loaded with Pyrodex pellets and a Minie bullet, and failed to fire. The owner evidently carried the loaded gun around with him in his truck for about three weeks, hoping to find someone to help, before somebody sent him my way. It took about 90 seconds to pull the bullet and the pellets with the ball puller from Track. The second gun was an antique that had "something" in the barrel. This turned out to be rags filling the bore for about half its length. These were harder to remove than the bullet, but an old-fashioned worm did the job with multiple passes. The third gun was a used T/C Hawken, a pawnshop acquisition, that was found to be loaded when I got it home. You don't want to shoot a "mystery load" out, because you never know but some fool might have put in smokeless powder. I poured in about a tablespoonful of WD-40 and let it sit for a few minutes. This lubricates the bore and (hopefully) some seeps around the projectile to make the powder charge inert. The Track of the Wolf ball puller screwed right in, but I ended up clamping the rod in my bench vise (with leather pads) and pulling on the barrel. I don't know how long the bullet had been in there, but it had made a home for itself. However, once it got started, it came on out. The projectile appeared to be something like a Hornady Great Plains Bullet, and there was a massive powder charge of some sort of very coarse, black granules. I couldn't positively identify it, but just washed it down the drain of my laundry tub.
The conical projectiles mentioned above were likely just a bit under bore size, I.E. not as tight as a patched ball. However, I have pulled patched round balls from my guns after "dry balling," and the ball puller has worked. I would like to point out that if there is a known or unknown powder charge under the ball, you are dealing with a loaded gun, and it's a good idea to not linger in front of its muzzle. I've never owned or used a CO2 discharger, so can't comment on these. I've been thinking of getting one, though.
Best regards,
Notchy Bob