Why wouldn't an Art unit not work in reserve? The trick is to have a lot of them there so the odds go up that they get used. Most players barely build them at all and most of the rest use them improperly.. they are Support Units you can move about independently, not front line combat units. They are best used mobile, moving from one trouble spot to the next.
AT Brigades can do much more to stop Axis thrusts than, say a Rifle Brigade, in some cases and both cost the same in CPs though the AT Brigades are cheaper in "stuff" and manpower.
I put the AT Artillery in Reserve, directly behind the defender.. minimum 9 at a time.
I attached a screenshot showing a 1942 AT defence, though in this case it was located only a few hexes behind the front, rather than as a rear area door stop.
The German XIV Panzer Corps was trying to do an exploit through a line that had been knocked loose a few turns earlier and was caught in the open.. I foresaw their route and placed a Panzer Trap in their path. There was one Infantry Division in each of the red X'd hexes where the Panzers are now, backed up by nine AT Art Brigades. None were entrenched.
The Germans initially hit with a 2 Pz Div haisty attack against the loan division but failed with loss! They then had to make Deliberate Attacks to make progress. I did not stop them, obviously, but stopped the deep penetration dead in it's tracks and inflicted over 100 tank combined losses in exchange for some cheap AT guns lost and/or routed
I don't have a screenshot of the losses screen but will upload one when I do, if you wish.
