Alright, here's the final rule regarding payloads:

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16: On a completely different subject, but still highly important; ammunition counts. For guns, I follow a strict system of 700 rounds for a 20-23mm cannon, 600 for a 25mm and 500 (attack aircraft such as the A-10) or 300 (fighters and multiroles, such as the MiG-29's and Su-27's) for 30mm's.
Regarding missiles, you can have 25 normal's on fighters, 30 on multiroles and 30 on attackers (all trainers carry 20). For special weapons, you can carry how ever many your plane can actually hold (realistically speaking).

On this subject, we have a wonderful explanation given by the famous Professor Silverwind Blade;

[i] During the Aurelia-Leasath war, many new technologies found their way onto the battlefield. As one of many continuous conflicts across the globe that had seen an increasing focus on airpower during a conflict, and the fielding of increasing numbers of aircraft, along with a rapid rise in advanced electronic and propulsion technologies, the very real need for aircraft to carry an increased amount of firepower was felt.
As a result of this, a crash-program was commenced to develop a new type of missile that would be physically smaller and weigh considerably less, but pack the same - or at least, a comparable - amount of destructive power into a smaller physical shell, and also be capable against both surface and aerial targets. 
The program was named akin to the Small Diameter Bomb, and took the name SDM - Small Diameter Missile. 
Using components from the Stinger AAM, Javelin ATGM and the Hydra series of rockets kept development costs low, and allowed for rapid mass-production of the technology. The technology was quickly adapted for export, and the self-contained tracking-and-guidance systems meant that the SDM could be integrated readily with a wide variety of aircraft for little cost and time. 
Taking over as the primary missile system on most aircraft in a short time, the SDM complements rather than replaces existing systems, relegating 'traditional' missiles to more 'special weapon' roles. Most aircraft can carry up five or more 5-tube 'packs' of SDMs, with some able to carry even more. [/i]
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Questions?