Think about it this way:
The weapon itself takes 30 seconds to form, which is significantly more time than it would take to reach for one's rifle.
It takes 10 seconds to effectively reload the weapon once it is depleted, which again, is a lot longer than it should take to swap magazines (yes you could just swap magazines the traditional way but that kinda defeats the point of the weapon being built over a fabricator).
So all in all you have a personnel assault rifle that is slow to draw and refills itself over time - which, to me, seems more reasonable than something that fires "...a wide mix of energy types (basically a full EM spectrum)... " out of a bottomless magazine.
And I mean it isn't firing anti-mech death farts out the barrel, just caseless, electronically fired, rifle rounds.