I'm pretty sure it's different with each weapon. As long as the weapon gets better then I don't care. I do know they get different scopes though.Does anyone know what exactly each upgrade does? I just know that MK 4 gives you a scope, and every 3rd one after that gives you a different scope for the most part.
http://www.se7ensins.com/forums/threads/best-upgraded-weapons.1283221/Can someone make a thread about what weapons are like upgraded to 25? Would be curious to see what all these weapons are like after completing the Easter egg.