I think that an RPG isn't a good idea. Most of the people think, that people will use it to grief. That's right!
So, what about changing the RPG into M202 Rocket Launcher? Well, it doesn't shoot rockets that explode like RPG, but the rockets make small explode, and set everything around on fire, because it's a flame thrower. It will have 4 rockets in one "clip", and you are going to have 3 clips. You don't understand? Watch this old video, that is probably older than you.
Video Here Click Me
Now you understand? I think this would be better than RPG. It won't destroy blocks, but set them on fire and hurt people.
The semi should be changed too. Like a Bolt Action Rifle? It would have 6 bullets in the magazine, and shoot a lot slower than now, but deal more damage. Like two shots in the legs or arms.
Smg, should have a longer reload time.
The shotgun is good now, don't really need changes.
What do you think? Please don't say "Meh, unbalancing bla brk afkio sucks fwanio". I think, this is a good way to balance the weapons.