anything launched which goes off on impact is a huge no-no, timed however is fine from a legal standpoint, it's really not all that much different to fireworks. The problem of course is getting a site to let you use them!
Actually, such a thing exsists, it was being sold on zeroin for £300. Seriously. OP got heavily criticised for it though on his YouTube channel as he linked it on his thread. , It looked incredibly dangerous as it launched pyro a fair distance at a high speed.