Q: What are masterwork rolls?
A: Most legendary weapons are capable of being Masterworked. The masterwork system is a way for you to upgrade your favorite weapons to their maximum potential.
Q: What does it take to upgrade a weapon to a masterwork?
A: Upgrading your weapons, in broad terms, takes a small amount of glimmer, legendary shards, and, most importantly, Enhancement Cores.
Depending on what Tier your weapon is at, the requirements to raise it to the next tier increase the closer the weapon is to being fully masterworked (Tier 10).
Q: What benefits do I get for masterworking a weapon?
A: Each tier that you increase your weapon will grant a small buff to a single stat, chosen randomly when the item drops.
Beyond that, at Tier 5 you gain access to a kill tracker. At Tier 10, your multi-kills will generate orbs of light.
Q: So what are these "masterwork roll" stats?
A: These are the possible stats that upgrading your weapon will grant a bonus to. Depending on what tier you are at, you can grant anywhere from 1-10 additional points in any of the stats listed to your weapon.
Q: Any catch with this?
A: The Bungie API currently says that all weapons can potentially roll with all stats as their masterwork stat. This obviously isn't accurate, as Blast Radius isn't applicable to, say, Auto Rifles.
The list you see on this page is trimmed to only show stats that actually appear on the weapon. However, it is possible that Bungie has additional logic behind the scenes that further filters these
possibilities to, for example, prevent certain items from being able to have an Impact masterwork. Until Bungie modifies the API files to 100% accurately display which masterwork stats are possible on
each item, take what you see here with a pinch of salt.
In raids and most dungeons, this will be strictly worse than Firing line because your teammates are usually near you in a well and you'll gain the bonus instantly.
For strikes, solo content, or some odd raid encounters it'll be able to be more consistent and in solo content it'll actually be able to go off.
Still a good perk, better off on special and heavy weapons rather than primary weapons. Firing line will usually be a better choice, but is still easy to proc when combined with a divinity or an enemy(s) that have easy crit spots.
"+20% damage for 10 seconds after landing half the magazine's worth of headshots *without reloading or swapping*" sounds pretty simple, and while that's true for 1) weapons with small magazines or 2) times where you're standing still and doing DPS on a stationary target, trying to use Focused Fury when neither of those criteria are met is a hilarious and painful wake-up call for your aim (that is, to say, it shows you that your aim is bad very fast). You're welcome to try it, but if you're not getting it on a Sniper Rifle, Linear Fusion Rifle or Machine Gun, it will grant you no mercy. What's worse, if you're not good at using this perk but force yourself to use it in a stressful situation first without practicing, it can make you get tilted, which comes with getting tunnel vision (not the trait) trying to activate it, which will just get you killed or cost you matches, etc.
...Unless it's on a weapon with Reconstruction, Subsistence, Overflow or the like. The number of hits is "just" a number derived from the weapon's base magazine size (after magazine perks/mods modify it), meaning that traits that refill the magazine without needing to reload give you more time to activate the perk - and if you activate it without misses, more time to use it without reloading. +20% damage for 10 seconds is... I don't need to explain that part to you.
So go ahead, try Focused Fury, but you're going to learn something about yourself in the process.
Even then it has to compete in the damage slot with perks that are more consistent and overall more damage.