I'd say that the Barrel mount is already full and can't support any more, Depending on the weapon, you still have a Top, Side or Under Barrel slot.
I thought that it was clarified/agreed that if there's an accessory listed there, it takes up that location and no further items can be mounted there.I also thought that when it came to the Gas Vent and similar items, you only applied the highest possible modifier when calculating Recoil Compensation.
Well I think that is the "intent" & we've discussed it to no ends here.
But in this case I was looking at the actual RAW now.
Which as I'm reading in the Mounts section means anything that comes with the weapon descriptions is considered to not take up one of the slots.
As for GV, I'm wondering about that because some say its from an actual GV product while others, Ares Alpha & Raiden for example, call it a "special chamber" system.
So again, purely off RAW Pg. 417, until errata'd, all the "crap" in the descriptions are essentially "freebies" right ?