Doing something for a weekend is a beast of a totally different color. Any sport/martial arts match lasts for an hour or two. Even if you spend an entire day playing airsoft or paintball, you also have to take into consideration that this is for 3 days. 12lbs doesn't seem very heavy and in truth it isn't, but carry that 12lbs around on your arms for hours of the day, over the course of 3 days as well as running (there's a lot of that) fighting and other exertions and you'll be panting. Throw in real armor and you're looking at another 15-50lbs (depending on what you're wearing) distributed all over you. Again, totally your choice and if you can do it, good on you mate. Another thought is that wearing metal armor in July also raises your chance for heat exhaustion substantially, if you haven't fought in armor in heat before (not sure if you're in SCA or other similar organization) you might not be aware of what your limits are. I've run around in chain mail before, and holy C@*# is that stuff heavy when you're running. As the others bought up also, the safety issue is the most important thing to worry about. But if your armor has no spiky bits then you should be ok.