For bullet build, firepower/close range/long range is all you want to see on armor. For AR, best bet is to use a blue preorder gun converted to tactical (save-scummed to 370+ firepower if you really want to min-max it) and level it up as you go. If you don't have preorder DLC, just cycle through whatever ARs you get as drops, converting each to tactical and always keeping Bone Shrapnel on once you can craft it. I wouldn't upgrade a random drop gun since you have no idea whether it will be any good at 50. Once you get to 50, the ideal thing is a tactical AR with ~95k damage, weapon leech/long range/close range, and Killing Spree + Bone Shrapnel for mods. Odds of that occurring naturally would be like winning the lotto, so you'll have to save scum to have any hope.
As for armor, I used pretty much all blues with the correct stats and upgraded them as I went along. You're limited to only 5 mods that way except on rare occasions where you get a good epic drop, but upgrading epics isn't sustainable without a lot of grinding that is time better spent on working your CT up to 15. I completed Archways on CT14 gold with all blue armor and only had to upgrade my AR to epic because I just couldn't kill everything without Killing Spree on top of Bone Shrapnel. It was a bit frustrating due to only having 5 armor mods total, but not as frustrating as getting to 50 and needing 216000 drop pod resources to upgrade all my stuff from 49 to 50.
(You can also use the mod exploit if you really need to. If you put Toxic Lead on your armor, then swap it for something else, you keep the ammo restore effect until you die or exit to lobby. Works with Perpetuum, Brain Eater, Vamp Mag, and most other bullet mods too. But not Trick Up the Sleeve.)