Rifles
A gunman knows how to keep his cool and has the reflexes to take the best possible shot.
The ability to use long guns like Assault Rifles and carbines, shotguns, SMGs among others. Capable of having very high damage output compared to most other weapons.
Advantages
Rifles are very, very strong, same as pistols. Dodge also seems to have little effect on tohit, making dodgy mobs and players alike cry, and they're mostly PC-, which means they're unparryable. Rifles also have access to specialized ammunition such as Iceblox and Thumper rounds, which do nigh unsoakable explosive and cold damage, and if paired with a crafter or using a crafting build, rifle users can have nearly unlimited access to hand-loaded rounds, which do much more damage than standard rounds. Rifles also tend to cause large amounts of damage, and bullet damage itself is only soakable by specialized armor or exosuits, generally. Rifles, as with pistols, also have the advantage of being able to attack from cover, forcing melee users to spend time charging at them, and greatly helping you dodge enemy gun users.
Disadvantages
Just like it's a good thing, most rifles, like other guns, are PC-, which means you need to rely on your dodge or a friend to not to get hit. Cover is hard to utilize most of the time for PVE, seeing how most mid-late game enemy automatically attacks you. And just like real life, guns need ammo. Carrying a lot of ammo is a necessity, to be able to go on long stints away from automats and gun stores. The specialized ammo is also very hard to find, either from the Oil Refinery or from Camp Benjamin, and generally only small amounts of it can be gotten compared to standard/handload rounds. Likewise, hand-load rounds require a decent crafter who has familiarity with guns to make them. The top-tier assault rifles themselves are difficult to acquire, as well. Guns also take time to reload, which can cost valuable time when in the middle of fights. And gun mods also require quite a lot of money compared to their melee counterparts. Finally, guns lose a bit of durability every time you need to reload, so you either need to be best friends with a repairer or have good skill with it yourself, or have to keep managing your ammo between fights, making combining your best friend.
Weapons
10/22 Rifle - Uses .22 speedloads, only useful for very early on.
American 180 - Uses .22 Pancakes, have fuck loads of ammo between reloads, your ideal grinding weapon.
EMP Shotgun - For hunting ghosts, considering rifles have no elemental mods. Can be loaded with either EMP clips, or waster shells like other shotguns to kill everything.
Pump Shotgun - Very slow and poor damage compared to most other rifles, as per usual with shotguns. Can be loaded with Flechettes, Waster Shells, Incendiary Shells, Slugs or standard buckshot.
Riot Gun - Another shotgun, a step above the Pump Shotgun, but still very slow and low-damaging compared to actual rifles. Can be loaded with the same shells as all other shotguns, except EMP clips. They're also PC1, which means you can actually parry with them.
Kel-Tec KSG Shotgun - The best shotgun, high capacity, quite a bit more damaging than the Riot Gun, and can be loaded with standard array of shells.
M100 Rifle -Can be bought from Blastarms in Redneck trailer park. Very good for grinding, considering it goes through 1-4.
AK-7 - Comes in both rusty and normal versions, very good damage output and uses the fairly standard 7.62mm type ammo. Can be switched to burst or full auto, and is used to make the AR-17 rifle. Rusty version is liable to explode while in use, so just get a real one.
AR-17 - Craft only, made from an AK-7 and other parts. More accurate and damaging, but quite a bit slower. Can still use the 7.62mm round.
SCAR-L - Former king of the rifles. Lightning fast, uses somewhat common 5.56mm round, and when loaded with handloads can outdamage the AR-17. Can be switched to single, fully auto, or burst.
Vector Carbine - An okay weapon all around. Uses VERY cheap and common 9mm ammo, and can be used from 4-6 raw rifles to grind. Comes from craft.
Needle Rifle - Uses needle clips. It's bleed damage gives it a niche roll among the rifles, since very few things (nothing whatsoever) can soak bleed.
Tommy Gun - Complete garbage, only good for decoration. Uses rare drum-magazines.
TEC-9 Machine Pistol - Drops from Karnivore Seniors, very very rarely. Most reliable way to get it is to run around and headbutt seniors until one of them passes out, then try to take it from them, or hit their hand and hope they drop it. Other than that, a fairly decent mid-game rifle, using the extremely common 9mm clip.
Rusty Gremlin SMG - Same as the TEC-9, albeit liable to explode and much more common.
Bullpup Rifle - An okay rifle, stands in the midground between the SCAR-L's speed and the AR-17's damage. Uses 5.8mm rounds.
Sighted Stermgewher - A fully modded Bullpup, gotten from Sky Raiders. Still uses 5.8mm rounds.
Plasma Rifle - The most powerful rifle in the game, the only reason it's not more widely used is because of it's expense. Made from craft only, and the ammunition is also craft only, and highly expensiveat that. Best used as a bosskiller weapon of sorts, considering the expense of the ammo.
Harpoon rifle - The rifle version of the harpoon pistol. Same damage, albeit a a good bit slower.
Strategy
Use Handloads whenever available, they far far outclass standard ammunition. If you have the rep, special ammo can be very useful (better than handloads) as long as you know which one to use where. Take cover before every engagement to make every enemy take time to hit you, or lower their tohit dramatically. Make sure to never run out of ammunition, and manage your ammo between fights unless you want to repair your gun all the time.