Unfortunately most gun builds are squishy and require a lots late game gears to shine, if only for main campaign you can create a self build (purifier or tactician are good for newbie), its rather fun than follow a guide.
I’m still a newbie at my first char.
I choose this guide as guidline and so far it worked quite well with self found stuff - currently I’m lvl 100, act 4 ultimate.
It’s a caster build, so no gun, but ranged.
Go 2h firestrike arcanist+demo. It’s easy to gear up and very fun to play. Just farm Crowley’s hide to get blue set and you’re ready to go for endgame farming. You can check my profile on this forum, I posted there simple build around firestrike. I play 100% SF and it was very fun to level up that FS build but… I don’t have time to level up it for AoM so this char is still at 85lvl. Take Care!