<TANGENT>
It was never taken. It was never a loadout option. The vast VAST majority flew with 8 guns and 267 rpg. However, keep in mind the physical space was larger. The oft-cited "500" (or in our case they got 425 somehow) is theoretical based on space. Even in the earliest state-side testing on B and C models it states "this was ballasted with 300 rpg but we report this will not be the load, and the combat load will be 267 rpg with 6 or 8 guns" -- its on almost every test report that cites 300 rpg or more. Nowhere, ever, has it EVER carried the "500" that is touted on wiki and other sources.
267 was the order of the day. The math on after-combat reports shows some varried in the range of 280-to-310 rounds per gun range, but these were less common. You would find 300-ish RPG on planes loaded with only 6 .50cal guns, as well. Outside of those, 267 was what it took. It's even painted on the gun loading doors on many models (as a text stencil) saying the max load is 267 rpg.
When the focus started shifting from pure fighter to attacker as well, the weapons were prohibited from having more than 267rpg (which was the standard anyways) if ANYTHING was on the underwing racks. When anything was on these racks, acrobatics and hard manuevers were prohibited as well. You could fly level and make dives to release your ord. Strong as it was, that's a lot of force and could seriously damage the wing.
Then in the Pacific you had the P-47N. This plane was tasked with a lot of fighting. Not only was the range used for bomber escorting, but it was loaded out with bombs and rockets to attack ground targets. When this happened, the plane was SERIOUSLY overloaded and could barely get off of island runways. Even when the runays were prepared. It then had to clear trees at the end! Many P-47s flew with even less ammo and guns, 4x .50cal and 200 rpg have been mentioned, but specifically many flew with only 2x .50cal and around 200 rpg to save critical weight. Even so, some fighters killed a good number of Japanese planes after dropping their bombs on target. The guns were strong enough.
P-47Ms were stripped down speed demons. To get the most out of them I've read in more than a few sources over the years that they only had 6x .50cal guns and reduced ammo. They were more than enough to kill with and the performance gains are noticable. From the lightest gun load to the heaviest in AH, the weight difference is nearly 1000lbs. Food for thought.
What we have in-game is a gross mis-use of historic loadouts on the P-47s.
</TANGENT>