This is off the top of my head, as if I was at the table right now and it came up on the fly . . . .
The way I would do such a disarm is by targeting a Trait. In the example in the book of targeting the Sentinel's Blaster d8, the player creates his pool and rolls against the Sentinel's pool with the Blaster d8 also tossed into it. The Watcher generates a total, as does the player. If the Player's total wins, and his Effect die is greater than the Blaster d8 rating, it's toast (if the E.D. is lower, the Blaster d8 is stepped back to Blaster d6 from taking at least some damage).
Okay, so I would say to target the trait as normal, and get an Effect die at least equal to the weapon die rating to disarm it. If you only step it back on that attempt, the narrative could say the two of you are wrestling for the weapon . . . or whatever the situation dictates.
Spend a Plot Point on a second Effect Die at least equal to the rating of the Weapon to create an Asset: effectively, you picked-up the weapon and can now use it in your next pool (otherwise, he's still disarmed but you just have a narrative gun). However, to be fair, the Weapon die remains the same. If it is a Gun d8, you have an Asset d8 created (even if you used a d10 or d12 for it). Even though a narrative game, the gun doesn't become more powerful (and the die step increases do represent gaining more power in MHR, as the ratings in Powers and Specialties show in the descriptions). What makes the Player's shot hurt more (or less) than when Mook #1 was shooting would be the Effect Die he generates when using said gun-- as that causes the Stress. Hitting with a d10 effect die represents a better shot on the target than a d6 effect die.
If the Asset is a single use, narrative can cover why. Maybe the hero runs out of remaining Ammo (if the mook gets it back-- if he survives long enough-- narrative says he had the clip to reload). Maybe the hero takes his shot and tosses it aside (cinematic fashion) and goes back to how he fights best. Again-- the narrative can cover the story reason.
(Note: Since technically the Trait was successfully targeted to disarm it [instead of destroy it], if the Asset was temporary [or none made at all] I would say the Watcher could spend a Doom Die at least equal to the rating of the Trait to give it back to the NPC. If not . . . then the gun was tossed away, or whatever makes sense for it to be gone from the character.)
If the player spends another PP, though, the Asset becomes persistent. Fire away until the end of the next Action scene. Remember, though, like any Trait, a Persistent Asset can also be targeted.
NOTE: I thought Assets had to be given to other Heroes to use in their pool, though. If so, then use the second effect die to create an Asset as normal (it can be of any die size, not limited as above). When you hand that die to another player, you aren't using the gun to directly attack the targets, but to provide cover fire to set-up your buddy's attack.