In the US, it is not legal to drop objects from amateur high altitude balloons. That includes launching projectiles from them.
Laws relating to high altitude balloons vary from country to country, so I don't know the reason for people not doing it outside the US.
Speaking of laws, at the minimum some things you should probably check before sending up a balloon is your country's laws on:
• What notification you have to give, if any, and/or markers you have to place on the balloon so that it won't interfere with air traffic as it passes through the altitudes airplanes and helicopters use.
• What methods of communication with the balloon are allowed. Many in the US us GSM cellular devices hooked to GPS units, but from what I've read that it is not legal to use cellular devices from high altitude balloons.
The best approach in the US is to get a Technician class amateur radio license (and if you are the kind of person who can put together a decent payload for a high altitude balloon, you can learn enough to easily pass the radio license test in a couple evenings). You can then use amateur radio frequencies and equipment for your telemetry, and you are even allowed to build your own radios. A transmitter can be pretty simple and cheap to build if you are willing to transmit your data in Morse code (like, a transistor, a crystal, and a handful of resistors and capacitors and such). If you are sticking an Arduino or Pi or some such in there, that can easily convert the data to Morse and send it, and automatically decoding it on the ground with a computer is not hard.
Everything else I've seen seems to be either illegal, or involves using unlicensed bands where you are legally limited to very low power which could be annoying (especially if your balloon lands far away from where you expected, and you are trying to track it down with radio direction finding). When you make licensed use of the ham bands, you can use a good amount of power (but only up to what is necessary to accomplish the mission).
In the UK, from what I've read, the ham option doesn't work. Hams are not allowed to transmit from aircraft, and that includes balloons. Not sure what the best legal method of operating there is.
• What constraints there are on the balloon. In the US there are regulations on, among other things, total weight of the complete system, total weight of the payload, and on the size/weight ratios of the components of the payload, and on the strength of the ropes attaching the payload to the balloon.
> What methods of communication with the balloon are allowed. Many in the US us GSM cellular devices hooked to GPS units, but from what I've read that it is not legal to use cellular devices from high altitude balloons.
Also worth noting, consumer GPS chips won't work above a certain altitude (or speed for that matter), due to weapons export restrictions to prevent them from being used in missiles, etc
Some consumer chips will work. The regulations say they must not work at speeds above 515 m/s and altitudes above 18 km.
Some manufacturers follow the rules as written, and disable if the unit is above the speed limit AND the height limit. Some disable if the unit is above the speed limit OR the height limit.
Laws relating to high altitude balloons vary from country to country, so I don't know the reason for people not doing it outside the US.
Speaking of laws, at the minimum some things you should probably check before sending up a balloon is your country's laws on:
• What notification you have to give, if any, and/or markers you have to place on the balloon so that it won't interfere with air traffic as it passes through the altitudes airplanes and helicopters use.
• What methods of communication with the balloon are allowed. Many in the US us GSM cellular devices hooked to GPS units, but from what I've read that it is not legal to use cellular devices from high altitude balloons.
The best approach in the US is to get a Technician class amateur radio license (and if you are the kind of person who can put together a decent payload for a high altitude balloon, you can learn enough to easily pass the radio license test in a couple evenings). You can then use amateur radio frequencies and equipment for your telemetry, and you are even allowed to build your own radios. A transmitter can be pretty simple and cheap to build if you are willing to transmit your data in Morse code (like, a transistor, a crystal, and a handful of resistors and capacitors and such). If you are sticking an Arduino or Pi or some such in there, that can easily convert the data to Morse and send it, and automatically decoding it on the ground with a computer is not hard.
Everything else I've seen seems to be either illegal, or involves using unlicensed bands where you are legally limited to very low power which could be annoying (especially if your balloon lands far away from where you expected, and you are trying to track it down with radio direction finding). When you make licensed use of the ham bands, you can use a good amount of power (but only up to what is necessary to accomplish the mission).
In the UK, from what I've read, the ham option doesn't work. Hams are not allowed to transmit from aircraft, and that includes balloons. Not sure what the best legal method of operating there is.
• What constraints there are on the balloon. In the US there are regulations on, among other things, total weight of the complete system, total weight of the payload, and on the size/weight ratios of the components of the payload, and on the strength of the ropes attaching the payload to the balloon.