What kind of rules would govern the use of a M45 Quadmount, if any? It's essentially four M2HBs mounted in a turret, which individually do 2d12 damage. While rules for turret weapons and autofire are readily available, there's nothing I can find that would indicate an M45 Quadmount would be in any way superior to a single M2HB. Damage of firearms would seem to be dictated wholly by their ammunition, and there do not seem to be rules for any kind of weapon with excessive autofire. Even the M134 Minigun is mechanically identical to most other weapons of its caliber.

What do?

Never used the system myself, so I figured, what the heck, I'll take a look.

Just looking at core SRD from Wizards, it all seems to be balanced by fire rate/Ammo capacity outside the damage.  The machine guns using linked ammo, assault rifles using mags with limited ammo.

Therefore, you are probably right, it would not be different unless you want to brew up something like dual wielding rules perhaps?

Yes, it sounds like what you are felling the need for would be an excessive auto-fire option house rule.  One very simple solution that would stay in flavor of the given rules would be to have the quad turret affect a 20' by 20' area.

Otherwise, options could include, Re-rolling and adding when a die comes up max.  Or increased crit range on burst fire, or option for increased damage if someone botches or fails there Dex check vs. auto fire damage by five or more.

The larger auto fire area would be the simplest, and would be in line perfectly as an application of the written rules.  Would use up 40 ammo, and cover 4 adjacent 10'x10' blocks.