Hmmm. No, i think those are Las weapons as i see no ejection port on them. (Yes, i know technicaly Bolters are supposed to be caseless, but we all know GW art ignores that because tons of ejected casings looks cool in art. ) Probably a specialized version for shorter range engagements, thus being shorter and more stocky than normal Las-rifles. This goes with the heavier armor we se them wear, so probably a close assault specialist regiment. (Or at least section.)
As for the caseless thing, i´m pretty sure i read it in one of the gamebooks way back, and an explanation in a copy of White Dwarf made by one of the top dogs at GW. I freely admit i could be remembering wrong. Been out of the hobby for years, unfortunatly. :/
They are based of old, unused (but compleatly AWESOME) Jes Goodwin sketshes. Those Roman cavelry masks ruin the desine a bit (but then I realy like milidustro style gasmasks, hazmat gear and life support systems) www.dakkadakka.com/s/i/gallery…
Don't worry, they're just vat-grown clones that have been lobotomised, heavily augmented, and fitted with mind-jacks that makes them an extension of their master's will. Just remember not to feed them after midnight.
I usually have my eye on too many spess mehreens to have IG regiments stand out to me a whole lot - at least short of something like, say, the Death Korps of Krieg - but the robes and masks really grabbed me. Officially my favorite regiment now, and you did incredible work depicting them.
Actual battlegear, I think, though it does not add any autosenses or the like. These folks, if memory serves, are devout warrior zealots and are emulating their matyred namesake with the masks. As for this name...cannot recall offhand XD