Alright, so, for some reason I can't edit my post, whatever.
I've fiddled with the stats of the weapon a bunch, and I've discovered the ammotype is to blame. For some reason it functions fine when swapping it to 7.62.
I will report back when I find out why this is.
Edit:
So I've discovered adding the tag '9mmGun' to the tags, like so:
<property name="Tags" value="weapon,ranged,holdBreathAiming,reloadPenalty,gun,barrelAttachments,sideAttachments,smallTopAttachments,mediumTopAttachments,stock,magazine,drumMagazine,firingMode,selfLoading,bottomAttachments,attPerception,perkDeadEye,9mmGun,canHaveCosmetic"/>
fixes the issue.
Frankly, I find this ridiculous, but it is what it is.