I know I've brought this up before, but would it be too much to ask for you to design these arm weapons with some consideration given to the people who will want to cram them onto the shoulder mounts?

My thinking is that (if making two different versions of each weapon is not on the table), maybe you could could have the armor cowl (which tends to cover the lower half of the receiver on each type of gun)as a separate part, and have a mounting socket on both the upper and lower sides of each gun. Then, for mounting on the arm, the cowling part would cover the lower socket. With shoulder mounting, the cowling part wouldn't be used, as the shoulder armor suffices.

I realize you are going the 'appocalypse rules' route for these things, but I would point out that FW doesn't limit itself to current rules (or the practical limits of ammunition storage).

Actually, Forgeworld's rules are the same as the Apocalypse rules, and neither allow the larger weapons to be fitted to the carapace.

Take the three pieces displayed at the top of this image.
Mount the shoulder armour into the joint socket on top of the arm weapon.
Chop the shoulder mount off the upper body piece, using a piece of styrene rod, or thick wire, or any other round object, pin the weapon onto the body as in the bottom picture.
Tadaa, shoulder mounted main armament.
I reckon it's going to be a fairly simple conversion without need of special bits to make things fit.

Well, hell, that seems good enough for me. Good job!

Another option is to include some beefed up targeter or some other technical doohickey running along the length of the barrel. That would make the overall part look bigger, but not upset your vision of having the main barrel having a size comparable to the shadowsword model.

Speaking of the Plasma Destructor: WOW cool design!   But i would prefer the three barrels closer to each other. They look a bit to far away from each other for my tastes.

In regards to the Plasma Destructor:

3 x Macro Weapon 2+, 45cm range? or 60cm?

Design wise - the model I showed is just E&C's blastgun chopped up, rotated and glued back together, nothing special, just 30 seconds of play.

The third/lower one. I guess it's the same length as the Gatling thingy?