If you combine weapons using (W1^n+W2^n+W3^n)^(1/n) rather than as a simple sum then you will not see very large distortions caused by additional (secondary) weapons.
I think that the 'effectiveness' of a weapon should be related more to the actual observable in-game effectiveness - here neither AAAW or LAW are as universally useful as their AArm FP suggest due to short (or very short) effective ranges - the AAAW also suffer from poor penetration, while the LAW have low accuracy.
Because of this, I find the near-parity of a 50 cal to the significantly more powerful ATG to be misleading - rate of fire cannot really compensate for low effectiveness... that even the KwK 42, KwK 43 and PjK 80 have AArm values only 1/3 more shows that effectiveness is not being clearly displayed.
I'd suggest that improving the method of calculation could provide comparable values for medium calibre weapons of normal performance (ie the 75mm mid velocity weapons), while modestly enhancing the presented values for high-velocity AT weapons, and limiting the value shown for LAW and AAAW types (or obsolete AT guns).
It wouldn't change how the engine handles firing within a firing step, but it would present more useful information to the player (and possibly AI?).
Basically my thinking is that a suitable combination of range, penetration, accuracy, rof and weight of shot could be produced, with the individual terms raised to exponents that gave a useful 'single value' effectiveness.
Poor quality AArm weapons would not have more than 1/2-1/3 of a 'normal' medium calibre gun, and the best weapons would show individual results around twice this value, for example.
Again, combining the firepower (or Armour) into the unit total, rather than simply summing the weapons would better present the quality of the equipment, and the likely results of an open-field battle, but this is less vital than not simply adding the secondary weapon to the tank.
(Eg if a M36 individually scores 32, and a Sherman 75 only 20, you will need more Shermans to "match" the AArm value of the 90mm guns... but to "match" a 5 vehicle TD platoon, you might need more than the 8 vehicles a simple sum would suggest - perhaps 13 or 14... of course it may be that tactical circumstances mean that 5 Shermans would be adequate to do the task, and the excess firepower of 5 M36 is wasted... or that no matter how many 75mm guns you have they will be inadequate...)
(eg from the test data a few posts up...)
50 cal 7.7 AArm
75mm Gun 8.8 AArm (using Chaffee Data)
Simply adding them - 75mm + 50 cal ~ 16.5 (which matches the Sherman 75mm data)
combining using 2, 0.5 as exponent pair:
(7.7^2+8.8^2)^0.5 ~ 11.7 (or the 50cal is reduced to ~ 50% effectiveness when combined with the main gun)
I feel 7.7 for the 50 cal is overstating it's usefulness even when mounted in isolation though - and a tentative estimate of 5 which I derive from the parameters in the Estab file using a sum of the products of accuracy, range, penetration, rof and shell weight (each raised to an arbitrary exponent) gives:
8.8+5 = 13.8
(8.8^2+5^2)^.5 ~ 10.1 (or the 50 cal in combination adds only 26% of the nominal value).