The phrase "one's primary weapon" refers to the most important or effective tool, skill, or attribute that a person relies on in a particular situation or in their daily life. Full definition
This will bring more interesting battles using primary weapon types remove the reliance on sniper camping and shotgun rushing. (mygameslounge.com)
I finally got all 3 that are legendary with an «melee at enemy» effect but do primary weapons come with the «shoot at enemy» effect? (blog.us.playstation.com)
The benefit is that your character's increase in experience gains, best primary weapon unlock and also its Basic Training perk. (thetech52.com)