I should have added 4 Formidable as an alternative in the original post; I've edited it. Always wanting to take something else, but always wanting to take it to get early armour and blast protection.
I'm inclining towards trihero's build now.
Gunner
Grazing Fire - Shredder - Demolition - Chain Shot - Cool Under Pressure - Saturation Fire - Rupture
More drawn out thought on all this. I really wish this forum had some sort of spoiler tag.
I wish more of the descriptions had ammo consumption and cooldown values. Can look in XComGameData_SoldierSkills.ini and XComLW_SoldierSkills.ini for some, but not all there. Until I learn different, I'm assuming most skills match their namesakes in the Long War Perk Pack. There's information at this link.
https://docs.google.com/spreadsheets/d/ ... edit#gid=0
Skills sometimes buff existing things or add something to a weapons' tactical toolkit. Adding a significant new thing tends to be more attractive. In both case, long cooldowns make the power seem not worth it, unless that single use in a battle can be gamechanging.
2 Shredder shreds armour (and is only available on the primary outside of AWC skills for the Gunner, Technical, and SPARK) and every hit should shredder, especially with 1 Grazing.
2 Hails of Bullets provides sure hits against high defence and high dodge targets, but uses 3 ammo and has a 5-turn cooldown.
Grenades and rockets provide both sure hits and shredding too, but the rockets especially are in short supply. However, both my common primary-focused Technical and SPARKs will be taking Shredder, as well as those who get it from the AWC. Do I need it on my Gunners as well? Well, when armour rolls out, it's going to be a pain. My Legendary campaign has had double-armoured drones since late March. In April has at least 1 double-armoured Muton on every mission. Which means he's ground-zero for the opening rocket.
But what when there's more of them?
2 Lockdown and 4 Mayhem make suppression reaction shots hit more often and hit harder. But that can come from having better aim and more powerful weapons. And the suppression isn't just to damage but to make it harder for the enemy to shoot back or move. The alternates, 2 Shredder, 2 Hail of Bullets, 4 Chain Shot, and 4 Formidable add somethings that weren't there before. 2 Hail of Bullets does have a long cooldown.
3 Iron Curtain provides an AoE attack with snare effect. Helpful against multiple enemy and melee enemy. But it's only available every 3 turns and costs 3 ammo.
4 Chain Shot and 5 Cyclic Fire boost the damage rate by delivering more shots in a turn. Chain Shot fires once and then if it hit, fires again, every 3 turns. Cyclic Fire 3 times every 4 turns. Good for momentary bursts.
5 Cool Under Pressure adds something new. It makes overwatch and suppression shots hit more often, but it also allows them to crit. If the Gunner can crit during overwatch and suppression shots, more damage is possible. trihero has said he always regrets passing up Cool Under Pressure on a solider later in the game. As well, 1 Grazing helps 4 Chain Shot get its first hit so it can try for the 2nd.
I'm thinking I'm going to have to test some of these skills and perhaps not just a quick run with a small team or just the solder and see how they work.