Boss Gitsmasha said:
Game mechanically, Legion Plasma guns are one level less rare than Legion Boltguns. Does this mean Legion plasma flasks are easier to acquire or refill than bolts, too?
Actually, yes, but for completely different reasons.
Astartes bolter shells are machine-tooled munitions produced in extremely limited quantities by select manufactories under contract from the Adeptus Mechanicus.
Fuel flasks for plasma weapons contain hydrogen, the single most abundant element in the universe. Of course, its refined, and higher grades (such as those used in Astartes plasma weaponry) will likely contain particular isotope mixtures, but it's still hydrogen, and thus infinitely easier to obtain than specially-manufactured solid munitions. The rarity of plasma weaponry is more to do with the difficulty of manufacturing and maintaining the containment systems (flaws in which can result in catastrophic failure) than because of rare ammunition.
Shotgun shells, even Astartes-use ones, are still shotgun shells - metal, casing and propellant. An Astartes one will be a bigger gauge than those used in mortal shotguns, certainly, but the ammunition is still fundamentally far simpler than a bolter shell.
Writing Credits for Fantasy Flight Games: Into the Storm, Edge of the Abyss, Battlefleet Koronus, Hostile Acquisitions, Black Crusade Core Rulebook, First Founding, The Jericho Reach, The Soul Reaver, Only War, The Navis Primer and Ark of Lost Souls