May have something to do with forceAllLinks, unsure though.
Beta build will be provided to assist with testing
https://github.com/boot2big/immersive_vehicles_vanity/blob/main/out/Immersive%20Vehicles%20Vanity-1.12.2-2.3.0.jar

Video showcasing actual VS expected behavior (Black&gray KARR is expected as it is gun-on-vehicle, white car is the actual gun-on-parts behavior)

Make sure to use the Devom Custom kit to mount guns onto a part! This requires you to open the hood to access and/or load the guns. (Or alternatively you can test this with the license plate slotted Weaponized Mounting Gear part, which takes the same missile launcher and dual machine gun. This part suffers from the same issue.)

P.S: Sorry for not telling you that I was going on break, I was not there to respond to your message. Unfortunately it did not occur to me, plus I had forgotten to tell various other PAs as well. I understand that this is how things will be going forth though...

This appears to not be an issue with the gun on the licence plate though?

Try using this with gun on the Devom Custom? Small note, you will need to open the hood to install and/or reload the guns.

edit: It just occurred to me that having two guns on one part may do it so I'mma test that myself as well, since it looked like you only had installed the machine guns.

P.P.S: Kicking myself in the head for forgetting a feature Laura suggested, with having separate fireable guns depending on if they're in front of back...

Okay, weeird. It works now, it shoots normally. I have no idea where to trace the route cause at this point, so maybe mark this as invalid for now... I swear it didn't work earlier though.