How to not be weak AF

I feel like I've played a decent amount of Endgame, and I still have weak AF firepower and get killed way too easily and am always by far the weakest guy in the crew. What am I doing wrong? I've got a combat rating of 20, skill track berserker at 12, tactician at 3 and gunner at 1, I've been using the Irides One with a bunch of attachments but I just switched to the X9 Maverick...while I see some dudes have RPGs with unlimited ammo just blowing shit up or full auto machine guns just shredding foes. I feel like I'm missing something. And then my crew will take off to areas with combat rating 50+ or whatever, and I don't last a minute and can't clear any objectives or get exfiled on my own.

I just feel like everyone else knows what they're doing and I'm missing something or holding my crew back having to always revive me. I've played Zombies, Endgame, the main campaign...and I'm still always the weakest guy in the crew.

Any ideas?