J.E. Sawyer said:
QUESTION: WMX/E pulled off new mods a hell of a lot better than GRA did. Seriously, how hard is it to just make the old weapons moddable, rather than spamming a bunch of new ones that are EXACTLY THE SAME?
ANSWER: The engine used for Fallout: New Vegas handles load order for .esm/.esp files differently on each platform (PC, PS3, Xbox 360). For purposes of this discussion, let's talk about the three that are relevant: FalloutNV.esm (the master .esm for Fallout: New Vegas), Update.esm (the patch) and GunRunnersArsenal.esm.
When we modify a form that exists in the base FalloutNV.esm and save it off in a new .esm, the base ID remains the same between the two so the engine understands that they are fundamentally the same form. When the engine loads the game, it loads the forms in FalloutNV.esm FIRST. After FalloutNV.esm is done loading, it starts to load in the other .esms. In this case, it loads Update.esm and GunRunnersArsenal.esm. If one of these .esms contains a form with the same base ID as a form in FalloutNV.esm, that form is overridden.
However, the platforms load these data files in a different order, and not consistently. If both Update.esm and GunRunnersArsenal.esm contained a form with the same base ID (e.g. a weapon that was previously patched), the engine may load the one in Update.esm last or the one in GunRunnersArsenal.esm last. If it loaded the one in Update.esm, it wouldn't matter that there were mods associated with the form in GunRunnersArsenal.esm.
In the not-so-terrible case, you get weapons that seemingly haphazardly have old stats/old mods or new stats/new mods. In the worst case, data in one .esm that relies on data in another loads first, attempts to reference the other data, can't find anything (because it hasn't loaded) and hangs or crashes the game.
On PCs, this is not an issue because there's a single platform and users can manually adjust their load order. For Gun Runners' Arsenal, creating GRA versions of weapons was the safest way to add items without generating a huge list of load order issues on different platforms.