Gun: Light Ammo
Why: Switching to standard Ammo adds an extra cost of 52 coins per 100 hits. That extra cash is better saved to buy a Sniper instead, which gives you way higher damage output for your money.
Rifle : Light Ammo
Sniper : Light Ammo (Preferred)
Why: You can use standard Ammo, but only if you're running a top-tier Sniper. Using standard Ammo on a lower-stat Sniper costs about 20 extra coins per 100 hits compared to a high-level Sniper running standard Ammo—and it only nets you a tiny 5–6% damage bump.
Tank : Blue Ammo mostly
Jet : Blue ammo or Heavy Ammo (Build & Aim dependent)
Heavy Ammo on a Jet isn't always a bad deal. If you pair it with the right build and have solid aim, it can actually end up being more cost-effective than standard Ammo.
I am not showing calculations cause it will make article unnecessarily long
P.S Heavy Bullets are basically a scam for almost everything—unless you're running a specialized Jet build.