I understand plenty the visual effect of Tatsuya and Crimson Prince shooting magic bullets unto each other and other magical shooting plethora of effects.
But it seems to me that all those spells could be cast without those gun-shaped CAD wihtout much fuss.
Is there any explanation to why there are gun-shaped CAD, from the point of view of an advantage to the magician, or is it just for visual dramatic effect?
As far as I know, this is mentioned only in the light novels, not the anime:
The Specialized CADs are often shaped in the form of guns as they have the auxiliary aiming systems incorporated in the area corresponding to the barrel. Co-ordinate data is input at the moment the activation sequence initiates, in order to reduce the calculation load on the user. Psions aren't emitted from the muzzle.
(Taken from the wiki, which takes it from Volume 1 - http://mahouka-koukou-no-rettousei.wikia.com/wiki/Casting_Assistant_Device_(CAD)#Specialized_CAD)
Omry is correct and so, to answer your question more fully, there is technically no need for them to be shaped like guns. The auxiliary aiming systems could be incorporated unto a differently-shaped CAD but, as you suspect, it only helps slightly to have it in the shape of a gun. That being said, even a slight advantage can tip the scales (not that Tatsuya needs it).
