The systems aren't incompatible at all though, there are full rules in ATOW for integration into BT. Even for aiming at the cockpit.
I know you can empty a fully automatic weapon magazine very very quickly, even a 200 round drum (I've done it, often!), but typical rifleman don't use full auto for that primary reason. Heck, the modern M16/M4 don't even have automatic settings anymore, just 3 round bursts (granted it still can easily blow through a magazine). There has never been a historic case of soldiers working this way, and I seriously doubt there ever will. It just doesn't make sense. If you fire full auto for 30 seconds of every minute, it won't take you but a minute or two before the weapon explodes in your hand. This is why modern weapons, though in many cases have rate of fire in the hundreds, sustained is typically far less. I've made a machine gun barrel start glowing red, and as soon as you fire a round in that situation and it doesn't leave the barrel, you basically get up and run as the weapon is ready to cook off.
In a combat situation it is *very* hard to hear, even with advanced communications. Guns and explosions are VERY loud, and to try to coordinate massed fire across a squad, much less a platoon, is difficult. It also means the squad is more vulnerable, after they do that once or twice the mech waits until it takes fire, then opens up knowing that entire squad/platoon is exposed shooting at it.
The weakest point on a mech is the cockpit, at BAR 9, and the only standard rifle that has a chance of hurting it is the Zeus, which if hit would have like a 10% chance of causing a single point of damage. Based on those odds, I seriously doubt anybody but snipers (much like today) would be taking careful aim. You have to stay still while exposed to aim, you'll be hard pressed in today's world to find a grunt to stand up to an armored HMMWV with a lowly AK, and in the 31st century the vehicles are FAR better armored, and FAR more deadly.
Based on the effectiveness of most weapons, and the way damage is resolved, it is pretty safe to assume that all the grunts are simply aiming center mass, with occasional rounds hitting all sorts of other things, just as any other weapons fire out there. Infantry can't even aim "high" or "low", but if they could they should be able to pick the punch or kick tables. The key thing here is we have no data anywhere in any source saying that infantry volley burst their fire like you described, but we do have ATOW which explains how the weapons work, how they interact with BAR values, and accuracy.
To put it simply, if we don't use ATOW values, and use the ones in the TM, a single grunt can carry a weapon that does 1+ points of damage to a mech. Based on the universe, typical infantry vs armor effectiveness, and simple logic, who would want to see that on the battlefield in BT?
On another note, I think the burst fire damage is basically the shots in a burst / 5. A bearhunter can fire 30 round bursts, this would be 1d6 additional burst fire damage, the battle armor version has 90 shots in a burst for 3d6. A Firedrake has 10, so its 1d6/3 additional damage, most of the machine guns do d3 or d4, so that seems to work ok. I'll add that to a new weapon list soon, which will have all the TM stats so people *could* use my list instead of TM, though from what I've seen there isn't a whole lot of interest in these new values :(
