The bread and butter of any First Person Shooter are the firearms that are available to you, and in Destiny's Rise of Iron DLC, this maxim is especially true considering how many possible firearm permutations there are. However, this wide variety of firearms does mean that it can be hard to determine which weapon is going to be your weapon of choice. Fortunately, there are plenty of options that are available to you that should fit almost any playstyle. Look back at Part 2 for Pulse Rifles, Scout Rifles, and Scout Rifles. Check out Part 1 for Hand Cannons, Sidearms, and Auto-Rifles.
Part 3 of this guide covers Machine Guns, Rocket Launchers, Shotguns, and Fusion Rifles. Due to their consistency and availability, the weapons featured in this guide are vendor weapons and thus cost 150 Legendary Marks.
Rocket Launchers
The Warpath
Available from: Future War Cult
Perks: Flared Magwell/Snapshot, Battle Runner/Heavy Payload, Tracking
The Warpath is a rocket launcher. It launches a rocket at something, typically resulting in the target being explosively reorganized into tiny little pieces. This particular version has Tracking, which makes it almost impossible to miss whatever it is that you're aiming at as long as they don't dive behind something. Plus it has a comically large blast radius.
Honorable Mention
Gjallarhorn
Available from: Being very lucky
Perks: Wolfpack Rounds, Quickdraw/Snapshot/Speed Reload, Tracking, Inspiring Jealousy
The most (in)famous weapon in Destiny's existence, the Gjallarhorn is making its return in the Rise of Iron DLC. Unlike other rocket launchers, Gjallarhorn essentially fires one giant rocket that explodes into smaller rockets, which also explode. Needless to say, this in itself puts it heads and shoulders above everything else in the game, limited only by its availability to players.
Fusion Rifles
Worlds to Come 001
Available from: Dead Orbit
Perks: Enhanced Battery/Snapshot, Performance Bonus/Hammer Forged, Hidden Hand
While it lacks the sheer power of Future War Cult's The Waiting, Dead Orbit's Worlds to Come 001 is much more well-rounded. Possessing a reasonable charge rate, fairly good perks for a Fusion Rifle, and a respectable amount of range and stability, the Worlds to Come 001 can be used in almost any situation.
The Waiting
Available from: Future War Cult
Perks: Accelerated Coils/Hipfire, Perfect Balance/Kneepads, Rangefinder
In terms of range, stability, and Impact, The Waiting is the best Fusion Rifle that is available to you in Destiny's Rise of Iron DLC. The charge rate, on the other hand, is so slow that it is entirely possible that someone can just slide into you and punch you to death before you get to fire your weapon. Obviously not as big of a concern in PvE, this fact should be kept in mind if you intend to bring The Waiting into the Crucible.
Shotguns
Last-Ditch 001
Available from: Dead Orbit
Perks: Flared Magwell/Quickdraw, Surrounded/Hammer Forged, Rangefinder
While the Last-Ditch may not even come close to Felwinter's Lie in its glory days, it does have two perks that increase its effective range. Needless to say, the further away from someone's hand you can be, the better, making the Last-Ditch 001 an obvious choice for both PvE and PvP. Unfortunately, the stock makes the weapon look like a shovel.
Machine Guns
Bonekruscher
Available from: Vanguard Quartermaster
Perks: Feeding Frenzy/Spray and Play, Quickdraw/Hand-Laid Stock/Crowd Control
The aptly named Bonekruscher may seem familiar to those who had the pleasure of firing some of Destiny's more powerful machine guns, namely Jolder's Hammer. Unlike other machine guns, this archetype of machine gun is capable of mowing through other players in just a couple of headshots, with the only real downside being its pitiful amount of ammunition. While the weapon's recoil may be off-putting, it is tolerable if you fire in smaller bursts (or even single shots).
Chaotic Neutral
Available from: Crucible Quartermaster
Perks: Feeding Frenzy/Guerrilla Fighter, Braced Frame/Armor-Piercing Rounds, Rangefinder
Virtually no recoil, long range, and a respectable rate of fire combined with a fair Impact rating makes Chaotic Neutral the most well-rounded machine gun in the Rise of Iron. Sure, your character apparently can't carry more than a few dozen rounds, but at least it can reliably kill most mid-tier enemies in PvE. Think of it as a poor man's Thunderlord in the Crucible, with roughly similar stats across the board.