The rules are an abstraction that take into account a large number of factors.
Assault 2 may indicate a light, nimble, highly accurate weapon which can be used easily even at a run and doesn't require a high rate of fire since the user can make every shot count - something like an Eldar Shuriken Catapult.
Assault 2 may also indicate a big, clumsy weapon with so high a rate of fire that the user is bound to hit at least a couple of times just be the law of averages - something like a Storm Bolter.
Don't think of the rate of fire listed in the rules as how many shots a weapon fires, but as how many effective shots it fires.
Note that this only works in the context of the game. In real life there's pretty much no such thing as a firearm that can be used effectively* whilst running.
*Depending on the effect you want - if you just want to keep the bad guys pinned then most anything man-portable and automatic can be effective whilst running.
Assault 2 may indicate a light, nimble, highly accurate weapon which can be used easily even at a run and doesn't require a high rate of fire since the user can make every shot count - something like an Eldar Shuriken Catapult.
Assault 2 may also indicate a big, clumsy weapon with so high a rate of fire that the user is bound to hit at least a couple of times just be the law of averages - something like a Storm Bolter.
Don't think of the rate of fire listed in the rules as how many shots a weapon fires, but as how many effective shots it fires.
Note that this only works in the context of the game. In real life there's pretty much no such thing as a firearm that can be used effectively* whilst running.
*Depending on the effect you want - if you just want to keep the bad guys pinned then most anything man-portable and automatic can be effective whilst running.
