With the arrival of Roboute Guilliman and the beginning of the Indomitus Crusade, Primaris Apothecaries didn't feel too keen on the standard Apothecary Reductor tool. You know that Chainfist-shaped thing on an Apothecary's wrist? That's a Reductor. It's a special medical tool used by Apothecaries as a way to safely retrieve the crucial gene-seed of their fallen Battle-Brothers so that new Astartes might be raised from among their Chapter's Aspirants, using a monomolecular saw for penetrating Power Armour and Ossmodula-enhanced rib cages, and a diamantine-tipped extractor drill. Retrieval and storage of a fallen Battle-Brother's gene-seed is so critical that Apothecaries carry a special tool for this operation, often included as part of the Narthecium. While a Reductor is not required for Progenoid removal, it significantly reduces the time.
While this may sound useful, and it is, the point of contention Primaris marines had is that having a piece of medical equipment permanently attached to your power armor could risk it being damaged in the heat of combat due to some numbskull trying to go all CQC on the Apothecary. So having the Reductor separate and actually become a weapon of sorts (like in that old vidja disaster Eternal Crusade) could provide the Apothecary a way to safely place this important tool somewhere where it won't get smashed.
The solution Guilliman and Cawl came up with is the Reductor Pistol. The Reductor Pistol is similar to the Narthecium's Reductor tool but takes the form of a pistol and is used to both quickly retrieve and store multiple-sets of Astartes gene-seed on the battlefield. The Reductor is designed to punch through Space Marine Power Armour quickly and cleanly; and can do much the same to the skull of a rampaging Ork Warboss if he gets close enough to the Primaris Apothecary. Although carrying the risk of being dropped, a pistol is much more easier to handle than a wrist mounted melee weapon and can be holstered to free up a hand.
On the tabletop the Reductor pistol has a puny 3-inch range, but boasts an impressive AP: -3 and D: 2 (though with only S: 4) if you ever get a chance to fire it. This will probably be never, which is less a fault of the Reductor pistol itself and more due to the inherent limitations of pistols: Apothecaries do not want to be in combat. They want to be safely bubblewrapped by their retinue or running around healing things. Though if you do get stuck in combat and don't find yourself immediately falling back, you have nothing to lose in firing it off since you can fire both it and the Absolver pistol into whatever charged you.
If you're wondering what happened to the rest of the Narthecium, it seems to be attached to a mechandrite on the Apothecary's back.