First off, BEFORE YOU DO ANYTHING ELSE, read this guide, courtesy of Steelpoint. It gives you a solid rundown of every piece of tech you'll ever trash wield as a specialist.
Since I'm a smartgunner by trade, I'll be focusing on that in the OP. I'm counting on players who are good with the other equipment to fill in for those.
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My hero <3
SMARTGUN TIPS
1. BACK UP. As a smartgunner you're the definition of a support role. The next asshat I see chasing a drone across the river with a smartgun and no backup is getting keelhauled underneath the Sulaco. You should be taking advantage of the fact that you can shoot through your friends CONSTANTLY. Relax behind a row of meatshields and fuck the xenos up, and then tell said meatshields to shield you as you load in another 50 rounds. The only time you're permitted to chase is if you heavily injure a target and you're SURE you can make the kill without getting harmed yourself. Even then, try to stay behind allies when you make the kill - likely they'll see it and want to finish it off, too.
2. Be mindful of attachments. By that I mean don't try to mod your smartgun to hell and back. There are only a few attachments that can fit on it in the first place, and almost all of them detract from the smartgun's best aspects like firing speed and accuracy. If you're feeling confident in your ability to land all of your shots, maybe.... MAYBE slap a barrel charger on there. But don't be that guy.
3. Do NOT grab that second power pack from the vendor at round start. I repeat, DO NOT. I see specs do this all the time, and it's wrong on all counts. Most importantly, taking a spare pack down to the planet at the beginning of a round (before encountering the xenomorphs) is considered metagaming and will get you into all kinds of admin-y trouble. But, on top of that it's a stupid fucking idea in the first place. You have no way to store that extra pack on your person, so in order to carry it around you have to drag it behind you. Dragging important items is one of the EASIEST ways to lose them. If you run in to trouble down there, you might as well be throwing 200 rounds down the drain, and you will DEFINITELY need them later on in the round. Make sure that second pack stays on the Sulaco.
4. GRAB A FLASHLIGHT. The M56 harness has no shoulder lamp, meaning you won't glow in the dark like your friends. While this might be okay for you, since you have the technology to see in the dark, your friends won't be able to see you at all - they could accidentally shoot you when they see that xeno run in front of you, or they won't be there to fight off that hunter that's dragging you away. If you slip a flashlight into your pocket, you're showing that you're aware of your teammates and your significance in the squad, and not only will it help you and your squaddies, but it will make them want to stick by you more.
5. Skilled smartgunners are the bane of T3 castes. Your high firerate will hurt hit-and-runners like ravagers before they can retreat and can chase off tanking crushers if you hit them at the right angle. Despite their toughness, most aliens tend to get VERY scared when they see such a constant stream of lead heading their way. Your ability to sit behind the melee and pour out a stream of bullets wears down and kills even the most fearsome and robust melee fighters. When faced with a foe buckling your front line, take notice of the amount of tiles away you are. As they move forward, you move back steadily. In the event that your bullets are being deflected by superior armor, try to move up or down the line to work that angled shot in.
6. By engaging in jolly co-operation with a fellow marine, you can bait sentinels, spitters, and praetorians out of hiding long enough to shoot them. Fire as they peek out from behind their resin walls. This will damage them as their spit is taken by the marine in front of you. Feel free to help up your marine pal afterwards, shoot down the resin walls together, and move forward steadily to repeat the same strategy deeper in the hive as the spitter falls back.
Any other smartgunners can feel free to add to these tips as well. If you disagree with anything I said... fight me.