I got two new sniper rifles from the gold key chest. One has about 7000 base damage. The other is E-Tech "consumes 2 ammo per shot" and has a little over 10000 base damage. When I test fire them (noscope headshots) at the dummy the 7000 base weapon does about 110k damage. Sometimes it also causes elemental damage over time. The 10000 base weapon does about 70k for the same shot.
The only thing I can think of is that my One Shot One Kill skill does not count on the E-Tech weapon, presumably due to "consumes 2 ammo per shot". I have 5+4 points in that for 108% bonus damage.
(sadly the 7k base damage weapon is Dahl and thus burst while zoomed)
Best Answer
From my experience of the game, consuming multiple ammo per shot does not affect One Shot.
In addition, if you look at the actual text, it doesn't say "first bullet" but "first shot" so, to me that indicates that the ammo consumed has nothing to do with the skill at all.
As a slight aside, I read this online and it helped me a whole bunch, try and get the unique gun "Infinity" which can be had by farming Doc Mercy. It consumes no ammo and so the game treats it as always having 1 shot in in the clip and so, every single bullet that comes out of there is treated as the first bullet from a fully loaded magazine. Works absolute wonders with the One Shot skill.