Bender. Ostap Bender.

The great combinator!
Lowered damage, headshot even may not kill the target. No charge-meter.

Let's A is a headshot, and B is a bodyshot!

This system with combos  (Inspired by another meaning of the word <<Combo>> from another (not mine) idea) can work this way:
For 3:
AAA, AAB, ABA, ABB, BAA, BAB, BBA, BBB. Only 8 Combos.
For 4:
AAAA, AAAB, AABA, AABB, ABAA, ABAB, ABBA, ABBB, BAAA, BAAB, BABA, BABB, BBAA, BBAB, BBBA, BBBB. Now - 16 Combos.

Possible ideas for the combos:
1) Next shot is critical.
2) Next shot will cover the enemy into the Jarate.
3) Next shot will ignite the enemy.
4) Next shot will cover the enemy into the MadMilk.
5) Next shot will make the enemy bleeding.
6) Next shot will heal Sniper for N HP.
7) Next shot will give the target a metal.
8) Next shot will give additional charge to the target.
50% of the combos can have zero bonus, 50% - as a one written above.

Headshot/Bodyshot made by you can be shown while you're scoping, so you always know what you can choose next, for the necessary combo.