What are the best modules for Heavy Gunner?

[Warhawx]

None of them really stand out for me, and a lot of em seem completely useless like hold breath

[BlackjackGT]

I usually have Field Rations and Take Cover equipped, but honestly, I don't usually find any noticeable difference after having tried most of what's available.

[neko_manma]

Routine is an excellent module.
Increases movement speed and armor penetration with each active reload, giving you mobility and firepower.
If you accumulate the stack up to max, mobility will be greatly different even when using the Onslaught System.
However, if the active reload fails, you lose the entire stack, so it is better not to do an active reload once you have accumulated the stack.

[⎛⎝Tiùwilo⎠⎞]

Routine can be cheesed a bit if you never actually reload your MG and just rely on onslaught to refill it. You could grind up the stacks with your secondary.

I do like On the Edge, as it can get some crazy health regen when combining it with Scarred. If you have the overdose with you, all you need to do is heal to 85% to cap out your HP then and there.

[HealthBar]

I take Routine and Press the Attack for mobility, in case I don't get Move Speed upgrade from the crates. Some boss fights such as Fighter Jet and Last Defender are a mobility-check and I don't like dying just because I got unlucky from the crate.

Though I recommend disabling Haste if you decide to take both, because Haste randomizes Active Reloads, and both Modules require Active Reloads to trigger. I turn off Flinch and Haste on 200% runs.