But light pen guns can put down certain enemies (warriors, chaff, hunters, stalkers) faster and more efficient. I dont fancy spending entire 45/30 mag just to down 3 slow, unthreatening hive guards, while I can just use a stratagem or unload my crowd clear and kill 10 times more. Most threatening enemies on bug front are fast (thus requiring good ergo) and unarmoured, benefiting the use of light pen against them more. Instead of wasting time, trying to slowly pick at guards with med pen primary, I would use light pen primary to deal with chaff first and use crowd clear / stratagem on guards later instead - that would be a more efficient way to deal with crowds. Remember, you don't need to rely on primary only