Here is a quick and easy way to recycle caulk tubes for re-use. Apply whatever substance you like in a caulk like manner even if you can't buy it in a caulk tube. Re-fill as many times as you want.

I wanted to be able to apply drywall compound from a caulk tube to do some touch ups on one of my projects. Nobody seems to make drywall compound or spackle in a caulk tube. Empty caulk tubes are also not readily available. None of my local hardware stores carry them. I found a few suppliers selling them online, but they were pretty expensive with shipping factored in.

The only supplies you will need are:
A caulk tube (preferably with water based contents)

Tools:
An air compressor with an air nozzle
Optional: bottle washer attachment for faucet

Step 1: Empty the Caulk Tube

Empty the caulk tube of as much of its contents as possible.

If you just want an empty tube to use, but don't have a spent one from a project, you can buy a new tube of painters caulk for $1.38 at Home Depot. Obviously, it is least wasteful to use the caulk for something rather than just throwing it out. But, if you don't need the caulking and need the caulk tube, this is the cheapest way to go. Buying empty tubes online and having them shipped will run you at least $3/tube.

Step 2: Extract the Caulk Plunger

You can extract the caulk plunger with a little compressed air. First, point the back end of the caulk tube somewhere you don't mind getting some caulking, such as into a trash bag. Or you could put a rag over the the back end of the tube. After that press the air nozzle of an air compressor firmly against the opening on the tip of the caulk tube. One burst of air should launch the plunger out of the back of the tube. Retain the plunger for re-use.

I also tried using a steel rod to push the plunger out, but the rod poked through the plunger.

If you don't have an air compressor, you might try one of those cans of compressed air. I haven't tried that so I don't know if it will work.

Step 3: Clean Out the Caulk Tube

If the contents of the caulk tube are water based, rinse the inside of the tube and tip with water until all contents are removed. I made use of my bottle washer which sped up the process and used less water. You can get a bottle washer like this from a beer making supplier. If you use one of these put your finger over the tip of the caulk tube to keep from spraying your ceiling.

If the contents of the tube are not water based, you will have to use the appropriate solvent to clean it out. This is probably not worth the trouble or expense, so I would stick to recycling caulk tubes with water based contents. You might be able to use a rag and a paint stirrer to clean most of the residue out also.

Step 4: Now You Have an Empty Caulk Tube

Fill this back up with whatever you like. I'm using mine for drywall compound, but you put epoxy, homemade Sugru (silicone + corn starch), or anything else you need to apply. You could probably even use this process to make custom colored caulking.