I've noticed several times that weapons I have equipped, but not using at that moment, will level up when I get a kill or complete an objective. (My secondary will level up when I get a kill with my primary and vice versa).
So do all of my weapons level up at the same rate once I have them equipped for a mission?
Best Answer
It depends on how you complete the mission and kill enemies.
Affinity (the exp used to level up weapons) is split amongst all your equipped items. When killing an enemy with a weapon 50% of the the affinity goes to the weapon used and 50% to your warframe. You also gain affinity when a nearby ally gains affinity, 25% of the gained affinity goes to your warframe and 75% is split evenly amongst the rest of your equipment. You can read the full rules regarding affinity here.