Lets sort it out, very briefly...I've said it dozens of times, and this is the best advice I can give for weapon unlocks.
Krylov: Stock weapon, massive destructive potential: when used properly, I take out entire squads if used in short bursts; it puts the Voss to shame at times, but panic fire still means you'll hit nothing but air. ROF is very high, so be cautious; SCAR 11 isn't so good, unfortunately.
Baur H-AR: Marksman rifle in single-shot, extremely effective at medium to long range, hindered by high but predictable recoil; at most distances, can take out support classes (read: important!) Must-have for those who like heavy guns with high damage potential--fair warning, you'd better sharpen your close-quarters skills if you use it, and be precise.
Voss L-AR: All-around good rifle for Assault: high ROF, suppressive, and dependable in short bursts up to 60 meters (and more, if you're good). Combine with Rifle rockets to dominate any distance--good solid unlock for prolonged firefights.
Pilum H-AVR: Engineer unlock that makes Engineer class merely camper to extremely agressive, when used in combo with Motion Mines. More damage potential in soft-spots of enemy armor, no lock-on tone, making it a silent kill. Be prepared to lead against moving targets, and it's no sniper rifle (deviation). One of my personal favorites.
Zeller-H ASR: Useless in my opinion, but can clear mines/explosives and 1SK light body armor (chest, torso or head shot). 3 round clip and long reload times means you WILL have to camp with it. Stock rifles allow you to be more agressive of a sniper, with larger clips and lengthier suppression time.
SAAW86: Useless unlock for air targets, get it last.
Lambert Carbine: Makes Recon agressive and Close-Quarters much easier (titan in particular) or defending areas, when used in conjunction with APM's and RDX. low recoil and can take stabilizer upgrade, damage potential is moderate.
Clark 15B shotgun: Close-Quarters master, aim for upper chest and head to ensure one shot kills at close range; will have to use other means to engage long-range targets as Support.
Ganz HMG: Useful for camping, the damage uppage from patch 1.10 is not to be underestimated: lower ROF means you will have to get in the first shot, better-used at longer ranges (Titan corridors, for example)
Please don't make me repeat all that. Hope this helps the newbies.