I have never tried to use equipment strapped along my forearm, but given that it shoots bullet-speed envenomed darts, I suspect that its default position should be neither under (in line with the palm) nor over (in line with the back of the hand). We move our hands around too much for those to make much sense.
I think the critter would find it easiest to avoid accidentally'ing the operator's hand (without having to rear 6" or more away from said operator's arm) if its muzzle was located on the "outer side" - i.e., along the specific forearm bone opposite the thumb's resting position. (I have never been able to remember whether that's the radius or the ulna.)
However lightweight and intuitively operable the wee beastie might be, I think that keeping it as close to the axis of the wearer's forearm as is practical would be pretty important from a usability perspective, as well as a "OUCH! I'm right next to you, watch where you're bumping that thing!" perspective. (That's a far bigger deal than most people expect; just try strapping something to your arm for a day when you're physically active and see for yourself. It's genuinely shocking how many things most people will bump, scrape, or narrowly miss hitting during such an exercise.)