I'll state for the record here
that I am both someone that has a great deal of experience with firearms including automatic military grade weapons AND someone that finds it to be a great waste of time and effort to try and model such a weapon's effects at the micro-scale.
Simply put, there are a vast amount of variables that can potentially come into play if one decides they are going to chase down the rabbit hole of micro-simulation of combat. This is true of hand-to-hand and melee combat. Especially true of combat involving firearms with a high rate of fire. Many, many RPG systems over the last 30+ years have attempted to do just this. All of them, in my humble opinion, have both failed to do so very well and added a daunting level of accounting, dice rolls, and overhead time to the game.
From a real world analysis of the effects of automatic weaponry in combat the real advantage of weapons with a high rate of fire (I am including everything from light machine-guns to semi-auto rifles and pistols here) is NOT damage potential to the target. Hitting an enemy soldier one time in the chest with a 7.62mm round generally leads to the same outcome of hitting that soldier with 3 or more rounds to the chest (and legs, arms, head) with a statistical reliability such that the outliers make little difference. The reason automatic (or multi-round burst) weaponry is so effective is because of their ability to pin down an enemy with sustained fire so as to both hurt their moral and allow for one to hold them in place to bring additional firepower to bear against them. In a close quarter environment, the high rate of fire allows one to engage and take down multiple targets rapidly (room cleaning).
These are both wonderfully modeled (or able to be modeled) by the FATE system. Having an automatic weapon gives you the easy justification to use that weapon in both blocks and maneuvers with a bonus. At least that is how I have ran it in my game. I treat "Automatic Fire" as a free taggable aspect of the weapon for those purposes if the character using it has a Good Guns Skill rating. Now, this is completely my call here and my players have agreed that it makes sense and works for us. I use a Good Guns rating as a threshold here because I know that using most automatic weapons effectively is not something that one can do easily. But, for someone that is skilled enough, this means that having an automatic weapon gives them a bonus for when they attempt to use those weapons to pin people down or other similar effects.
They already get the ability to make spray attacks. That can be a big deal even if it can be hard to pull off. But, as I have said above, hosing multiple targets with automatic fire
effectively is not something anyone picking up a firearm can do.
Between those two bonuses, I do not see a need to give automatic weapons a boost or any reason to try and model increased damage based on number of bullets fired. The place that it might would be against some of the bigger, tougher supernatural bruisers. Against those really nasty things that just wade through fire, at best you are using the weight of fire to try and slow it down. That is modeled as a Block with an automatic weapon gaining a +2 to the effort. Against something that bullets can hurt but are tougher than human, I would say that anyone capable of holding their sustained automatic fire on target (a moving target!) like that would have an appropriate stunt that would give them a bonus with an automatic weapons. An alternative would be to allow a very strong character to use his Might skill to supplement his Guns skill in that case. Strength does matter in the case of controlling an automatic weapon.