[Aranador]

All right - I will run with you on that being a second trigger rather than part of the grip.  But I can seriously only see a barrel for bullets, a gas return vent for the autofire, and a shorter wider barrel for a larger sub-velocity munition.  The only other thing out the front couldn't possibly be a barrel for anything more than a party popper, and strikes me as more of an optic for the smart gun system.  Even the existing under barrel looks a bit too short really, but probably OK for a base bleed propellant grenade.  Certainly nothing like a semi-auto shotgun though.  Heck, the block text even says it only has one secondary feed mechanism. suggesting it couldn't handle 2 secondary munitions.  I even suspect that you detach said feeder and any under barrel weapon to set up with the bipod should you want to use it with the sniper or LMG.   

Anyway - unless a dev wants to step in and state that it is one way or the other, I guess it is going to be a GM call.

[martinchaen]

I wouldn't put too much stock (heh!) in the illustrations...

According to the description of the weapon system:

The XM30 is H&K’s modular weapon system. Each kit comes with a shock-pad-equipped stock, smartgun system, imaging scope, two underbarrel secondary weapons, a secondary feed mechanism, and four different barrels along with a bipod for the heavy options. The XM30 is intended to equip entire military or police units with the ability to change roles based on the operational situation.

Note that it says it comes with an imaging scope and a bipod. Neither of these are pictured. To me, this makes it pretty clear that the two underbarrel secondary weapons, the secondary feed mechanism, and the four different barrel are NOT in fact all mounted, but are options.

[EDIT]
The attached is what I can make out from the illustration. And no, I don't believe you can have two underbarrels.

[Namikaze]

Actually, having the underbarrel mount be modular would help the balance issue a lot.  Additionally, I think it potentially opens the door for modular underbarrel mounts with that weapon - not in the game right now, but they could be house ruled pretty easily.

[Kincaid]

I assumed this was the Steyr AUG-CSL for 5th edition, so it's one function/one underbarrel at a time.

[Aranador]

Actually, having the underbarrel mount be modular would help the balance issue a lot.  Additionally, I think it potentially opens the door for modular underbarrel mounts with that weapon - not in the game right now, but they could be house ruled pretty easily.

Well - given that you could put a slide on any weapon with an under barrel slot, then keep a range of under barrel devices handy, the modular idea is already doable.

[BaronAtomy]

Boy, that's a bullpup only a mother could love.  Seems really strange to have the two triggers both on the main grip; no idea what the lowest forward-point on that thing is; flashlight?  Smargun system?  I could see the second being a large-diameter aperture for a grenade launcher/shotgun, or just a tube mag; looks like the gas tube is on top with the rail system running over it.

  Notice there's no 'these barrels are primary, these secondary' designations?  By RAI, I'm assuming you have the AR, Carbine, Sniper, or LMG able to be primary barrels and the SG/GL fixed as secondaries, but it does make one wonder where that feed system they talk about is.