SoH has it down right, but just to make it clear.
Jet packs let you fire rapid fire weapons as if you were stationary and allows you to assault. But that's only for Tau jet packs.
It doesn't matter how many shots you fire, If You Fire A Weapon that is RAPID FIRE, You May NOT Assault
. (Unless you count as stationary for shooting, like bikes, jet packs, terminators, etc)
If the target is within 12" you have to either fire two shots or none at all. You can NOT opt to fire one shot...and even if you could, you STILL couldn't assault.
If you moved during your movement phase you may ONLY fire twice at 12" or not at all.
You CAN fire a pistol once and then assault (This is why BA and DA come with pistols now), you can NOT fire a pistol twice and assault.
If you move at all, you may NOT fire your pistol twice, you may only fire it once or not at all, but you can still assault.
That about cover it?