I don't have any experience, but if you will allow for my chime-in...

They are an answer to to an un-asked question. Legal? Anything is legal until you are caught. That legality question should be asked of the BATF and not on some forum.
Safe? Until something breaks. Different ammo produces different pressures. How far do you want to push it?

If I desire a weapon in a specific caliber, I prefer to purchase one made specifically for that caliber.

 

I don't have any experience, but if you will allow for my chime-in...

They are an answer to to an un-asked question. Legal? Anything is legal until you are caught. That legality question should be asked of the BATF and not on some forum.
Safe? Until something breaks. Different ammo produces different pressures. How far do you want to push it?

If I desire a weapon in a specific caliber, I prefer to purchase one made specifically for that caliber.

What Dan said

 

I have a flare gun on my boat. 12 g. It is made of plastic. Made to drive a projectile that weighs next to nothing, with chamber pressure less than that. If I put a sleeve in the barrel, I still have a breech face made of plastic- and presure WILL be placed on it.

NOT one for the "good idea" bucket.

 

These are primarily used by the boating community.

Nobody realizes yet that they would make great arson weapons.

If you have your own 12 gauge shotgun, you can launch them through it.

If you have a handfull you could set an entire forest or neighborhood quickly on fire.

But like I said, these are primarily used by the boating community. I used to race yachts from Saint Francis Yacht Clubhouse to Monterey, and we always had one set onboard with us. BTW that's where The Americas Cup is now on display, thanks to Larry Ellison.

Anyone can buy them and possess them, however it is illegal to launch one over dry land.