This accessories is supposed to go on the barrel and cannot be cumulated with another vent option.

Did the Ares Alpha Special chamber allow it or not (gut feeling no) ?

Can it be mounted even if there is an underbarrel grenade launcher (gut feeling yes) ?

My group has always interpreted it as the special configuration adding more recoil without taking up a mount, so the gas vent 3 was an option (barrel mount). Of course, if your group feels 5 recoil with a grenade launcher is too beastly, raise the availability IIRC, there was some mention in the previous SR4 book about it's 'bullpup configuration' accounting for the extra recoil compensation.

The special chamber design has nothing to do with a gas vent so there's no problem.

In SR3 the 2 inherent recoil came from the bullpup design and the special chamber design (1 each). In SR4a the Alpha apparently no longer has a bullpup design but still gives 2 inherent recoil compensation from the improved chamber design.

In SR4 Bullpop does not exist at all anymore, as there is no building guns from scratch and no individual Concealability stat.

It doesn't have any special rules but it does exist, for example the Nitama Optimum II has a bullpup design.

I was refering to the fact that the picture of the Alpha in the anniversary book shows it with a classic design, not bullpup.

Bah. SR4A is good for rules and extra fluff, but the original is better for weapon art. The Alpha is a bullpup (magazine feed behind the trigger group).

Bullpup only really means you get the same gun in less length. As for the GL it's mounted under barrel that's a different slot. There's top barrel and bottom/undebarrel.

And stock, if you're using shock-pads.