As has been noticed before the weapon values don't seem to be quite balanced. I don't think the Bolters are too good though (perhaps you could tune them down to 2D10+4/2D10+8), I think plasmas, meltas and assault cannons are not even near where they should be. It would be crazy to give up the full-auto capable bolter (including suppressive fire) for a plasma gun. So a fix is in order.
[i should add that I don't blame FFG here - I have been expecting problems with that as that is not easy to balance... after all GW did some recallibration since 1st Edition too.]
I think we need new values for Plasmas, Meltas... flamers seem to be okay (or are they not?)... Assault Cannon... Sniper Rifle... and I didn't even look at grenades and missiles yet.
Okay, I'll try one or two ad hoc (I'll assume Bolter values of 2D10+4/8 here, if you don't like that, just add another 1 or 2 points to damage):
Meltagun S/-/- Dam 4D10+8 Pen 13 Special: Meltas add 1D10 to Penetration at short range or below
[comment: damage comparable to krak missile, pen higher]
Plasmagun S/2/- Dam 3D10+10 Pen 10 Special: Volatile
[comment slightly lower dam and Pen than Meltagun]
Assault Cannon -/-/10 Dam 3D10+5 Pen 7 Special: Tearing and a few specialties that have yet to be determined.
It's just a first suggestion... if we can build agreement on these 3 weapons, then we can extrapolize from there to the other weapons (pistol and heavy versions, etc.).
Alex