I think there are principles for all functional builds: crowd control, active & passive defense, diversified damage output...
IMO sniper rifile shall be backup by SMGs (smart rapid) and nades because 5 STR don't support most AR, and it's a simple calculation: you do AR burst, you don't have AP to use SR, vice versa. For a non-AR gunner build, I strongly suggest to take Grenadier at Lv4 and Spec Ops at Lv8, make 5/20/20 mech/chem/bio in early game for Toxic Gas Grenade, or S/L to get low Q shock emitter and plasma core.
Base:
5
6 (Grenadier, 30 point in Throwing is fine)
7 (Hit and Run)
3
10+++++
3
6+ (Gun Nut)
Opportunist, Aimed Shot, Ambush, Snipe, Sharpshooter and Critical Power are must.
Stack your crit chance with equipment.
Burst attack is still powerful because special bullets are powerful and they don't occupy your utility slot.
I remember I've seen a thermo-sniper build on this forum, but I can't recall when I saw it or who made it. Premeditation 189% Thermo-destabilization is dominating!