“I think Stewart and Nadir offer sound general advice, which I will repeat in a slightly reworded form just because I love chiming in on this sort of thing.
Gun placement has to be considered in the context of the scenario, first and foremost. Think of your setup holistically, with mutually supporting elements, a plan for every piece, and a piece for every plan.
As Stewart says, know what your opponent needs to do and how much time he has to do it. That guides your general line of defense – up front to disrupt or even stuff an attack under severe time pressure, or deep to influence the endgame of a grinding assault. Also consider whether these are primarily tank killers or anti-infantry pieces. Even light ATGs can do a number on infantry, especially as acquisition stacks up and the crits start to flow.
Location-wise, try to identify obvious choke points, such as a bridge he has to cross. That’s probably the right locale for a gun, but sometimes a spot is so obvious it takes care of itself, as the enemy deliberately avoids it (but beware of the Sicilian Dilemma!). Also, if he starts on board with decent smoke assets, promising up front gun positions will find themselves socked in and possibly with no LOS whatsoever.
Nadir touches on terrain – buildings and woods are nice because a broken crew has rally terrain at hand without having to abandon their ordnance. However, that comes at the cost of doubled CA change DRMs. Also, note some ordnance can’t set up in buildings – the Soviet 76L ART piece probably tops the list of illegally set up guns, since so many people think of it as an ATG.
I find players are a little too afraid of hindrances when they place their ATGs. A wide field of fire suffering from +1, +2, or even +3 hindrances is more effective than a limited but pristine arc. Take this into account when considering the gun’s vulnerability, too. If a position is substantially more survivable but suffers from an added hindrance as a result, it might be worth embracing.
I am not a big fan of aggressive ambush locations, such as an isolated orchard hex on an extreme flank. You might get devastating flank shots but you also might not even see an enemy AFV. Some monster scenarios provide enough materiel to support such shenanigans but small and mid-range cards typically demand a coordinated effort. Gambit placements will leave you short-handed on the main axis of attack more often than not.
Finally, an out-of-place gun remains an asset as long as it’s hidden – the enemy will obsess over where and when it will appear. However, that fear and anxiety might not be doing you as much good as actual 50mm APCR rounds, so don’t forget manhandling. As Nadir points out, you can hustle a gun into position relatively easily if the terrain allows, and adding a squad to help out will guarantee you move a lighter piece a couple hexes a turn.
Good luck, and have fun!”
(A gentleman asked for advice on Gun Placement in ASL. Mr JR Tracy gave it a brilliant writeup which I think bears repeating. So I asked him for his permission.)