I found the sweetspot now:
M99: 1250 damage and still a 1.1 headshotmultiplier, a 30% Fleshie resistance on Beginner, Normal and Hard, whereas a 45% resistance on Survival and HoE grants this:
* 1-shots every Clot, Stalker, Crawler, Siren and Gorefast on the body on all difficulties and number of players. This is both for Sharpshooters and Non-sharpshooters.
* 2-shots Husks and Bloats with bodyshots (Regardless of perk) on some of the higher difficulties/number of players. (Husk can still barely be one-bodyshotted on 6-man Hard, Bloat cannot)
* Scrakes can be 1- or 2-headshotted / 2-decapped by NON-sharpshooters depending on diff/player number.
* Scrakes will always be 1-headshotted by a lvl 6 sharpshooter on any difficulty and number of players. (This is balanced imo)
Fleshpounds will be:
* By a level 6 sharpshooter 1-headshotted on 6-man Beginner, Normal and Hard and on 1- or 2-man Hell on Earth and on 1/2/3-man Suicidal
* By a level 6 sharpshooter 2-headshotted on 4/5/6-man Suicidal and 3/4/5/6-man Hell on Earth.
* By a Non-sharpshooter it takes (on the higher diffs and player numbers) 4 or so headshots to DECAP the Fleshie, so it's kinda useless for Non-sharpies against Fleshpounds
Safe to say: The rifle is a Scrake-killer in any player's hand, but only a Fleshpound killer in a Sharpshooter's hand.
With these numbers either you should be able to carry a lesser sidearm (like a single Mk23) or you get more shots for the M99 as a Sharpshooter (i lean more towards the bolded & underlined part myself)