Courtney,
Thanks for taking the time to respond.
Firstly, my apologies for inadvertently inferring that you were an 'F-35 hater' - I did not mean that, I know you take a fair view on that programme. Sorry.
On calibre for air to air combat, I will respectfully differ, and I'll explain why.
The overall effectiveness of any gun system can be built up by multiplying probability of a hit (Ph) by probability of a kill (Pk). In some simulations, Ph is calculated for a burst, not a single round, similarly for Pk. In others, the single round values are used, and then Pk is 'added' (sort of) for a burst hit. In both, you could also add a ranging factor in to reflect how often the aircraft might be able to use the gun at all.
What I found in the 90s was that there were no standard models for calculating Pk, nor was there much reliable data around the Pk of various rounds. The best data available was from a series of live firing trials carried out in the 80s in Norway. Basically, they concluded that high velocity large fragments fired into the aircraft along the fuselage were the best 'killers'. (Interestingly, the Mauser's shell was optimised to blow large holes in wings when entering at right angles). What is (hopefully) indisputable is that a single larger shell has a better Pk than a single smaller shell. I know of no reliable quantitative models for assessing the Pk of a series of hits. I do know that the DERA/AWC models in the 90s were really badly wrong. (Mathematically wrong).
So you need to look at now the round or its calibre might affect Ph. Interestingly, gun rate of fire does not have a massive effect on this. The key factor is time of flight. Shorter time of flight (faster bullet), better Ph. (Bullet shapes were improved in all calibres in the late 90s, using data from Gerald Bull's superman studies). Another factor is the bullet density in the target's area of sky. Again, not directly linked to rate of fire, can be affected by dispersion. This is affected by many things, but at longer ranges, the faster the shell, the better.
Revolvers (and guns like the Gsh-301) reach their full rate from the first round. Gatlings take time to spin up. In a half second burst, most revolvers get more rounds away than a Gatling. (I understand that Gatling users sometimes compensate by using longer bursts, starting before the 'pipper' goes over the target - but this wastes shells).
So, in my view, and informed by the calculations being done in the 80s, 90s and afterwards, the larger (faster) calibres (25mm, 27mm, 30mm) are better air to air systems. I would point out that nearly all the newer systems appearing use these calibres. They also give the system a far better air to ground capability, with much longer range and better target effect. In short, this is why the F-35 went for a four barrel 25mm gun rather than the six barrel 20mm M-61.
I'd go further - looking at overall system weight, power demands, and volume, i believe that revolvers or reciprocating cannon (e.g. Oerlikon, GsH-301) are a better overall system for fighter aircraft use. I know that many will differ, but honestly, it's not all about rate of fire all the time.
That's my view, anyway., not that it matters much.
Hope some folks find this mildly interesting, if not sorry for boring you.
Engines