idk how it is done but i have mates running around with heavy weapons on a medium/light body type thier doing some sorta glitch with the smg idk if its fixed yet but it is something to look into i could see it being a problem in the future.

I have acedently got this once but then it was a light body with shotty and machinegun...

You can do it by being a heavy body type.

Assigning your 'Heavy Weapons' then changing your body type back to medium / light.

You take an XP hit but you gain the weapons you shouldnt have.

I noticed this also. I see alot of lights w heavyweapon, I was gonna post about this because currently my light has ar and smg equiped. even if u turn off the console u still have it. hope it gets patched.