[Akiyoshi]

Yet devices still need maintenance and can't alter mass at will as freely as you mention it. They just switch between stablished forms adapted trough engineery (unlike, say, Sakura's Magical Wand who adapt new unprecedented forms by the power of her growing will and magic).

Armed devices are even more evident in this aspect as those seem to be made of sturdier stuff than Intelligent ones (at least Generation 1 ones) and be more mechanical in nature, cartridges are probably made of real materials because they are solid shell roudns encased in boxes and more likely without a mana support before being filled it by Shamal/Shari/Mariel.

As we lack any confirmation i also have another theory as i credit the Device form-switch to a function more similar (similar, not equal) to the capsules made by Capsule Corp. in Dragon Ball which are capable of encasing all sorts of big physical stuff inside hammerspace.

Devices do have some mass-alteration trough macig, tough. As we've seen with Bardiche's Zamber mode, Raising Heart Excellion, the way Laevatein can alter the lenght of it's Schlagenform and Graf Eisen size-changing capabilities. Unless Signum and Vita are consumate liars when they say stuff like "tasting the steel of Laevatein" or "The iron count, Garf Eisen" i think it's pretty safe to assume their weapons are made of real metal. Which still makes sense in Laevatein's demise at the hands of Cypha as the latter's viralizing power is precizely to be able to weaken solid constructs, specially the ones made of metal xD