As already proposed in the ATM discussion thread started by @Joe Bopper, this is my latest approach for a fully quirked ATM boat for mid range engagements. Skill Tree proposal: https://kitlaan.gitlab.io/mwoskill/?p=jsonbin1.5993e61d1da63e05fbc64aee&s=Weapons
Although having more than 24k GSP lying around I tend to level new mechs the classic way, at least partially. Therefore I can't give you a comprehensive answer yet. After some matches I found ammo to be a bit short, so I switched to HSL instead of the HML in favor of additional ammo.
Ammo is short, yes. <20 salvos, with 30% Missile cooldown quirks. I'd try to get around 35-40 salvo's (25/0.7=35.7, and I try to equip 25 salvo's on mechs without CD quirks). Using missile rack skills, and probably by going down to a 12 and a 9 (or 2xATM9). This build has 36 salvo's with missile rack.
On my ATM9x3 Mad Dog, I bring 800 missiles and have managed to run out. With that cooldown boost, I would expect that ATM12x2 would need similar ammo levels
I find that ATM ammo count is very dependent on Range Discipline. If you wait until you can see the whites of their eyes 800 missiles is 2,400 damage. If you are firing at 550+ you are going to burn through ammo fast and have a much lower hit percentage.
I moved the tag to the left arm, removing 1 HML. Removed tonnage from my RA and that way mounted plenty of ammo for dual ATM12, while keeping the range and improving the heat profile! I have a recording where I played this during community drops in Baradul's stream. 1100 damage without strikes, just because I could fire down a hill with TAG assistance. I'll try to find the time mark.
This build is amazing. I've consistently topped 1k damage with my highest at 1226. Probably 6 of my first 20 games were over 1k with another 10 or so between 600-800. ATMs are amazing.
Made room for an extra ton of ammo downgrading the TAG and reducing armor on the associated arm, It breaks off pretty quickly but to me it beats sacrificing firepower.
I generally use a Light Tag with my ATMs as well. Because the range coincides so well with the ATM mid-damage range, it makes for a nice fit. You don't want to remove ECM on targets beyond 500 meters anyway, it'd be a waste of ammo to fire on them. You can put another 10 points of armor on that right arm by dropping 10 from the head. I know it's generally considered bad form to reduce head armor that much, but the Maddog head is almost impossible to hit, even by accident. Worth the risk in my opinion, to keep the tag arm.
Honestly not sure why I overlooked the head, it's usually one of the first things I strip for spare tonnage.