So, i was messing around on elite create a class and used inspect element on the mp5. a few lines down(the line highlighted in blue) it says how much gun exp you have on that level and how much to level it up.

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im thinking if you change that to something like "89,000 of 89,800"(which is only 800 exp to level 31) your gun will be level 30 on your game after you send the changes