If you are looking for the currently fastest boss-killing rocket for arc or void burns (since Gjallarhorn fills the solar niche), then you can look to the Pax Totalus EPR8 to fill that role nicely.
[url=http://www.destinydb.com/items/3881784492-pax-totalus-epr8]Pax Totalus EPR8[/url]
I suggest this easily overlooked rocket for that role because it has the highest fire rate of any rocket launcher currently available in Destiny. As everybody should know, the most important stats for a legendary pve boss-killing rocket are rate of fire, reload speed, mag size and total reserve ammo. (Velocity can also be considered if the boss is a significant distance away.) Since fire rate is non-adjustable across all rockets, the Pax's high fire rate allows it to expend its magazine faster than any other rocket launcher. With the highest base reload speed of any legendary rocket launcher, it can also ready itself for another shooting spree just as quickly.
To maximize this rocket's boss-killing potential, the recommended perk setups are listed below. The bolded perks are what to best go for when reforging, while the other perks are alternatives for when RNGesus just doesn't want to bless you:
Tier 1: [b]Warhead Verniers[/b], Hard Launch, Smart Drift Control.
Tier 2: [b]Tripod[/b], Clown Cartridge, Spray and Play.
Tier 3: Either [b]Speed Reload[/b] or [b]Flared Magwell[/b], [b]Javelin[/b]
Tier 4: [b]Tracking[/b], [b]Cluster Bombs[/b], Surplus.
[b]Perk Priority:[/b]
The perk priority when reforging is as follows:
Tier 4 = Tier 2 > Tier 3 > Tier 1
[b]Tier 1:[/b]
Increasing velocity is usually a top priority for most of us, so [b]Warhead Verniers[/b] or Hard Launch are recommended, with Warhead Verniers having slightly more velocity over Hard Launch. Smart Drift Control is for those who want the velocity reduced to its lowest possible when used alongside Tracking. (See [b]Note on velocity[/b] for more.)
[b]Tier 2:[/b]
Maximizing total shells per tube allows the Pax to make the most of its DPS capability. With [b]Tripod[/b], the Pax will be able to hold 3 in the tube instead of 2. This is significant for 2 reasons:
1. Provided you have some armor that increases heavy ammo capacity, Tripod allows you to hold a total of [b]8 shells[/b] (3 in tube/5 in reserve), one more than the Pax's ordinary count of [b]7 shells[/b] (2 in tube/5 in reserve).
2. Even with the Pax's fast reload, the number of reloads needed to shoot all rockets should be kept to a minimum to maximize DPS. With Tripod you'll only have to reload [b]2 times[/b] to empty all your reserves, while without it [b]3 reloads[/b] would be required.
Clown Cartridge and Spray and Play are both alternatives for if Tripod doesn't work out for you. Clown Cartridge gives a 40% chance for one extra rocket in the tube per reload, while Spray and Play dials the Pax's reload speed (when empty) up to 11.
[b]Tier 3:[/b]
The choice between a reload perk (i.e. [b]Speed Reload[/b] or [b]Flared Magwell[/b]) and [b]Javelin[/b] is dependent on whether you want to reload faster to expend you're reserves faster or if you want your rockets to get there faster. I think that a majority would prefer Javelin since the Pax does have a very low base velocity (which can affect battles against distant or
floaty bosses), especially since it already has such a high reload speed. Also, reload speed can be additionally patched with the Trials of Osiris gloves (if you get a lucky roll), while Javelin is the only significant velocity patch that the Pax can get (along with a Tier 1 perk). Of course, by using a reload perk you will be able to absolutely maximize its DPS, allowing it to reach levels not capable by any other current legendary rocket launcher, a niche only the Pax can fill. Fortunately, since the reload perks and Javelin don't occupy the same perk slot (reload perks are in the 1st slot of the Tier 3 column and Javelin is in the 2nd slot), it is possible to roll for both and freely switch between them as needed.
[b]Tier 4:[/b]
[b]Tracking[/b] is the vanilla option that you can't go wrong with, allowing for more accurate hits on distant or mobile targets. However, if you feel that you are able to place your shots without need for tracking (since some bosses tend to be more in-your-face then others), then [b]Cluster Bombs[/b] gets a special mention for being the only re-rollable rocket launcher perk to (inconsistently) add extra damage to large targets. The numbers may wildly vary, but there'll always be extra damage dealt.
[b]Note on velocity:[/b] The choice between having Tracking or choosing some other Tier 4 perk determines how important velocity actually is for maximum performance. Without tracking, maximum velocity is very important to ensure quick delivery without missing a mobile target. With Tracking, velocity becomes less important overall since the perk somewhat compensates for any deviation in the enemy's position. In fact, it could be argued that having Tracking with minimum velocity allows the rocket to curve more when homing in on its target, allowing it to hit closer targets with more consistency. The decision between these 2 possible setups is up to you, so think about where and how you'd want to use the Pax and go from there.
In terms of alternatives, Surplus is there because it basically doubles the amount of ammo you pick up from the floor for all your equipped weapons, allowing for abundant firepower from all your guns.
[b]Faults with the Pax[/b]:
Obviously the Pax has its faults, and looking at the base stats it can be easily seen what. Both blast radius and velocity are abysmal for a rocket launcher, making it so that even patching them up with Heavy Payload or Javelin still leaves it subpar compared to other rockets. As a result, the Pax can't be recommended for killing groups of adds since other rockets can do a better job.
Also, as mentioned at the start, the Pax can only be recommended during element burn boss fights (Arc or Void in particular). If there is solar or no burn, the Gjallarhorn is the best rocket launcher for the job. If you want to use a legendary rocket launcher to kill groups of adds or big adds as well as killing bosses, then the Radegast and Ash factory (amongst most other rocket launchers) do that job better. Also, the Pax can't be recommended for crucible due to its poor base velocity. Once again, rockets like the Radegast or Ash (and Truth for obvious reasons) do a better job there.
[b]TLDR:[/b] Overall, if you want the fastest boss-killing rocket for arc or void burns, look no further than a Pax with Tripod, a reload perk (Flared Magwell or Speed Reload) or Javelin, and either Tracking or Cluster Bombs. With the highest fire rate, it's a small niche worth taking advantage of. Maybe?
[b]Where to get:[/b]
The Pax can bought at the crucible weapons vendor for 150 marks if anyone's interested.
[b]Edit 1:[/b] 1 thing I forgot to mention:
If you have not been loved by RNGesus enough to get a Gjallarhorn or you'd like an alternative to the Hunger of Crota for bosses with solar burns (for some reason), then a Pax rolled with solar and good perks is probably the next best thing out there.
[b]Edit 2:[/b] Th0rG0d brought up an interesting point regarding whether or not the Pax's low stability causes the recoil per shot to be too much to take advantage of its fire rate when shooting a specific target. This compelled me to buy and max a Pax of my own. With Arc, Hard Launch, Tripod, Speed Reload/Javelin, and Tracking, I set out into a Strike to test it on a boss (The Nexus in this case).
The result: I found the recoil to be entirely manageable per shot. Since bosses tend to be large and only a moderate distance away from you, even with the recoil you'll still be on target. Additionally, even though the Pax has the highest fire rate of all legendary rockets, you should have enough time between shots to make minor aiming adjustments (even when firing as fast as the fire rate allows). You'd have to be shooting a shank at distances that would take a Timebreaker sparrow 5+ seconds to get to before you'd have to pace your shots.
[b]Edit 3:[/b] Added [b]Cluster Bombs[/b] as a major alternative to [b]Tracking[/b], and a note regarding the importance of velocity based on perk setup.
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Great right up! But, The pAx comes in different flavors even if you roll it to where you want it. I have had some amazing Pax's and some with the same rolls, but different stats in velocity, blast radius, stability. weird. the higher the tier your main Guardian is, the better outcome your rolls are going to be in your weapon (s).