We know the missions! Each mission has been designed to be either short or long range. This requirement is to make sure the weapon used is more suitable towards the type of mission. Primary guns with the required power will still be mandatory to play these missions, this is in sync to the strength of enemies you'd face in the mission. 

You could however, yet use the secondary weapons in these missions i.e., Teslas for Long and Heavy guns for Short.

This applies for PvP battles too. Although, there aren't any prerequisites for the weapon power. Snipers/ Teslas  for long-range battles and Assault/Heavy for short-range battles.

Note: Not all players can possess a secondary weapon and hence, the prerequisite.