I may need a some more help here. 
I currently have "Button 0" set for "Fire all guns", but when the secondary weapons select is drop tanks/rockets/bombs (no guns), only 2-out-of-6 .50-cal guns will fire in a p51. What I want is have "Button 0" fire all my .50-cals no matter what  is selected in the secondary weapons box.
no, it's either you are ready to fire rockets/drop bombs or fire all the guns.  I like it that way and I'll tell you why.
let's say I am chasing some guy and want to make him nervous, so i fire the primary which is 2 mg's just to get him nervous, instead of wasting bullets by firing all 6 mgs.  when I have a target solution then I will fire all 6 mgs.  this is only possible because we have 2 mgs set as primary and 4 as secondary.  same for the spitfire.  I dont want to waste canon so i do a quick tap of the mg's to make the guy nervous while saving the canon for when i have a good shot.
I wouldnt want to always fire all weapons all the time, it's a waste of ammo.  would you really like to fire all 30mm cannons in the 262 for a kill?  that would be a waste of 30mm.  same for the il2, I never fire the primary which is mg's to kill tanks it's a waste as it cant damage a tank and obscure the target, I only shoot the 37mm cannons.
if you drop bombs/rockets and keep forgetting to press backspace then it's a training issue for you.
semp