Why do we need to reset the bone first? Let's assume that bone_angle.pan is set to 30 degrees at a certain moment. Then, let's imagine that the player moves the mouse a little further in the following frame, until bone_angle.pan reaches 35 degrees. But the gun was already rotated by 30 degrees in the previous frame, so it would now rotate another 35 degrees, setting the pan angle to 30 + 35 = 65 degrees!

This is what would happen if we wouldn't use that "ent_bonereset" instruction inside the while loop. By using ent_bonereset we discard the old angles every frame, so the bone will only get the newly computed angles (pan = 35 degrees in my previous example)