You got it right Zonie. Just take out the nipple and tap a little 4f into the hole. Not too much, or it will compress under the nipple as it's screwed back in, and form a very hard, solid blockage that will not allow the spark to ignite it or pass through to the charge.
I have used this method to remove a stuck ramrod, but for stuck projectiles, I can almost always get it out with a ball puller. I was very leery about trying this with that ramrod, but I couldn't get it out any other way without damaging it. It ended up shooting about 30 yards and hitting some asphalt, which left a mark on the aluminum Knight range rod, but it didn