It may just be me, but I’m under the distinct impression that Arty’s ability is delivered via UAV like Skyhammer’s. Regardless of whether that small detail is true or not, the fact remains that his ability is largely useless.
Perhaps an interesting way to make him useful and raise his skill roof is to actually give him a significant call in time for his ability, something that you can activate via designating for the normal loading duration with three shots and then wait for around 10 or so seconds before the shots actually land with the ability to redesignate the location during this cooldown (and have the rounds actually have to travel the distance so you can’t do crazy across the map transitions with a half a second till impact) with a very powerful blast that is even stronger than Skyhammer’s drone shot for shot. It’s not the most realistic thing ever, but it would make him very deadly.
That of course is probably slightly too OP, but it’s certainly tweakable.