There can be many different approaches to a problem, and they might have different merits.
The best – IMHO – is the one, that grows out of the already existing, established games of the great&only GG.
The GG games, that I love, are:
War in Russia (WiR), Pacific War (PW), Carrier Strike (CS),
Battle of Britain (BoB), 12 o'Clock High – Bombing the Reich (TOH-BTR)
Don't tell me that you don't love these

.
In TOH-BTR and BoB, the guns have, besides range & penetration, also damage and accuracy (hit probability).
In PW & CS, the guns have separate flak rating, which is EFFECTIVELY hit probability modified by range –
that is, probability of a hit on a plane during the time, that a plane travels from max gun range
to the point of dropping bombs/torps.
IMO, the MINIMUM solution would be the following characteristics:
1. Range (exists).
2. Penetration (exists).
3. Damage (damage of a HE shell, used in bombardment of soft targets, like infantry) (exists in TOH-BTR & BoB).
To Dunedain: Damage combines damage of 1 shell & fire rate into 1 number.
4. Accuracy against ground targets
(hit probability when shooting at hard ground targets - ships, shore batteries, tanks, etc)
When using AP shells – damage is halved, as in PW & CS.
5. Accuracy against aircraft (hit probability when shooting at airplanes). ALWAYS less than #4.
On the reason, why Damage and Penetration ABSOLUTELY must be different,
see the old discussion in PW forum here:
showthread.php?s=&threadid=10870
If we take a gist of discussion – it's as follows:
The world is 3-dimensional. So, for a given technological level:
The DAMAGE of 1 shell is approximanely proportional to it's weight, slightly modified by shell velocity.
The weight is roughly proportional to caliber cubed (Cal^3).
The damage during a fixed amount of time is damage of 1 shell by fire rate.
Fire RATE generally tends to decrease with caliber slightly faster, than inverse proportionally.
That is, an 8in gun usually fires more than 2 times while 16in fires 1 time,
but 1 16in shell weighs about 4 times the weight of 2 8in shells.
Generally, damage is proportional to caliber squared (^2).
Although the lower-calibre automatic guns have rather high damage – because of high fire rate.
The PENETRATION of a shell is proportional to it's kinetic energy per unit of cross-section area.
That is, it is rougly proportional to caliber, STRONGLY modified by barrel length.
That is, 16in/50 cal gun penetrates about 2 times the armor that 8in/50 penetrates
(provided shells have same technological level and the armor is of the same quality).
The last (#5) parameter is like flak rating in PW & CS,
but it is NOT a summation of hit probability during ALL attack run, but during, say,
time it takes an average plane to fly 1000 yds.
So:
1. Faster planes will get a bonus (less time under fire).
2. Torp-bombers w/longer range torps and fighters w/longer range guns on strafing runs (P-39 w/37mm gun)
will also get a bonus – they turn away earlier.