Jammed charger removal and repair.
You cannot remove the charger from any of the charger models when the plunger is jammed in the down positon. This is not an uncommon problem on older cannons. It is usually caused because someone did not clean the Bangsite out when they put the cannon away. The carbide reacts over time and leaves a fine lime dust in the charger. Then someone forces the charer down and it gets stuck there preventing removal of the charger itself. In the photo below you can see where this poor charger has been abused to no avail. The good news is that this can still be saved.
The first thing you must do is remove the barrel from the cannon. Sometimes these are rusted in also and must be soaked with penetrating oil (WD-40) and tapped gently with a hammer handle until they are loose enough to remove. Once the barrel is out, make sure the charger is screwed all the way down. It is a good idea to soak the charger inside and out with penetrating oil. Then with a drift or long screw driver, tap the plunger back and up to its normal position.
When the plunger shaft is exposed for about 5/8" you should be able to unscrew the charger.
Once you have the charger out, remove as much of the lime as you can from the inside of it and then remove the plunger spring. With an oil stone or fine emory board, remove any nicks from the exposed plunger shaft. Then drive the shaft down and out of the charger body. Clean all parts and remove any other nicks from the plunger that may keep it from traveling freely. Make sure it moves freely in the charger body before re-assembly. The image below shows the proper installation of the charger plunger spring.