This is my suggested build - Triple dmg type, for this gun - it is my fav gun - Hema :D

If u guy feel hard to aim on target's head. Just remove Heat(6060) and use another dmg mod or crit mod. I just wanna show your about 10k heat DoT :D

From wiki:

Headshots will restore health to the player, equal to 10% of the damage dealt.

I'm not sure about Lifesteal on headshot but I think that it's true.

This weapon deal innately Viral damage. After Viral is upgraded with max bonus dmg 325% on target has 10 Viral status applied. BUT ONLY DAMAGE TO HEALTH.

From wiki:

The status effect of Viral damage is Virus. It amplifies damage to the health of the afflicted target by 100% for 6 seconds. Subsequent procs add 25% increased damage to health up to 325% in total after 10 stacks, with each proc having their own duration.

But we do not just fight vs Non-Armored unit, so we need a Universal way.

As you can see: Corrosive dmg is highest, it will mostly proc, then Viral and finally Heat.

When u shoot at target:
1) Corrosive will apply to strip Armor (max 10 procs, so 80% of initial armor will be removed)
2) Viral will apply to halt down HP.
3) Heat will apply to reduce 50% of CURRENT armor, also Heat now add infinitely Heat status, this status will deal damage-over-time.

This triple-status combination will destroy most of all target types.

From wiki:

When Heat status is combined with Corrosive status and Corrosive Projection, the armor reduction is multiplicative. This can be expressed as the following:

Armor Reduction = 1 - (1 - 50%) × 1 - (20% + 6% × Number of Corrosive stacks) × (1 - 18% × Number Corrosive Projections)

For example, a full 4-player squad with max-rank Mod TT 20px Corrosive Projections, an active Heat b Heat proc, and max stacked Corrosive b Corrosive proc will result in an enemy's armor being reduced by:
1 - (1 - 50%) × 1 - (20% + 6% × 10) × (1 - 18% × 4) = 97.2%.

If only one player has a max-rank Mod TT 20px Corrosive Projection with the above statuses, an enemy's armor would instead be reduced by:
1 - (1 - 50%) × 1 - (20% + 6% × 10) × (1 - 18%) = 91.8%.

An active Heat b Heat proc, and max stacked Corrosive b Corrosive proc without Mod TT 20px Corrosive Projection would result in an enemy's armor being reduced by:
1 - (1 - 50%) × 1 - (20% + 6% × 10) = 90%.