Using heavier BBs may lower the FPS, but that's not the problem. It's all about the joules. I was tricked by this, I thought you could just use heavier BBs and wallah, I can use my gun legally.. Well, no, things don't work like that. It's quite long to explain, well it takes me some time to explain, but if you want me to, I can try to explain it to you - bare in mind I will most likely explain it over 3 paragraphs aha.