If a ranged weapon has an internal recoil compensation of 2, can i monunt on it a barrel Vent Gas 3 to have 5 recoil compensation total?

Yes.

Depends on the weapon. If it can take the GV3, then yes - you can stack ReComp.

If a weapon can mount the barrel options, it can mount the GV3, right?

Pretty much.

Isn't Internal Recoil compensation an Internal Gas Vent ?

Depends on the weapon. Some internal comps are customized constructions, heavy barrels, personalized grips - it could be anything, though the description text will usually tell you what it is.

It depends what the internal recoil comp is.


For example:
A White Knight Ingram has internal recoil comp of 3. That's because it comes with a built in GV3. So, it cannot take an external gas vent.

But it could take a heavy barrel, front grip, etc.

If you see something with 3 points of RC internally, it almost certainly has a gas vent already.

Gas vents are restricted to MPs, SMGs, ARs, and machine guns.

As far as existing internal recoil compensation goes, as mentioned each weapon should specify exactly what the recoil compensation is, which could be anything from gas vents or a folding stock already installed to a freebie 'efficient design' bonus.

the recoil bonus from folding stocks, bipods, tripods, etc are always listed in parenthesis
ie:
if a weapon had a tripod it would list under recoil 2(8) representing that the weapon had 8 points of recoil comp when using the tripod.

For definitive rules on what RC stacks, see Arsenal, p148.