Those cylinders will take off and fly in any unpredictable direction if you pierce them, unless you are using something like API or APTI which will open them up and ignite the contents (obviously not going to happen with an extinguisher). And you want to be about 1100yds away from it, as I was.
Very inadvisable. You’re essentially creating an unguided missile. The contents are typically going to be at 300-400 psi, and if you do the math you’ll find that the tank can travel quite a way (and in some random direction) before the pressure drops enough not to propel it, not to mention spewing corrosive chemicals. As johnpost wrote, the proper disposal method is to take it to a hazmat disposal site, or call up the local fire department and obtain their guidance.
Stranger
Home fire extinguishers and the ones I have at work pressurize to between 150-175 psi, not 300-400psi
Assuming we are talking about a 1A10BC home fire extinguisher, while yes you would be creating an unguided missile the total amount of “fuel” (pressurized gas) is fairly small and the pressure is not that great. While I have never shot one with a rifle, I highly doubt that you would get over maybe 20’ of travel out of one.
If this was still a worry, hold it down with a couple of rocks then back off and shoot it.
In the interest of science I think I see a trip to the dessert in our future. 
Well, that’s why you put a chain around it first, anchored to something heavy. Then you can watch it spin around desperately like a leashed hummingbird before giving up.
I periodically replace my fire extinguishers. Thankfully, I’ve never had to use one in the house. When I replace one, I have used the “old” one for training purposes outside. My kids have learned on a small brush fire, what to expect when using an extinguisher.
Usually, the handle is just a lever that presses against a button or pin at the top of the cylinder, so you can probably use a stout screwdriver and get some leverage against the remains of the handle to press that pin and discharge the extinguisher. Or maybe just get a good grip on the cylinder, aim the nozzle away from you or anyone else, and lean on it, pressing the pin on a rock, brick wall, etc. Even if you only manage to get one <pffft> out of it, the remaining pressure will leak off in a couple of days.
Hmmm… I just happen to have a 5-B:C extinguisher here that claims to contain 2.5 pounds of Halon 1211 and dates back to 1987. I thought Halon had been outlawed, but I just had a look, and there’s a handful of Halon extinguishers available on eBay.
Or you’ll just run the risk of an unguided missile dragging a big rock or a chunk of concrete chained behind it.
It’s my understanding that you can’t buy a new Halon extinguisher, but you can have old ones serviced.
Well, in that case, definitely try it, because that be hellacool.
I was going to ask about disposing of an extinguisher once its pressure drops too low (i.e. unused but old), but it sounds like you can simply toss it in the trash - once it’s been fully discharged. However, if the chemicals are potentially corrosive and toxic, where can you safely discharge it?
See posts #40 and #43.
Stranger
Thanks. I did see the bit about hazmat disposal - which we don’t have that I’m aware of - but missed your line about the fire department. Seems obvious in retrospect and should have thought of that myself.