Regarding ammo efficiency.
Here are some numbers I've gathered. Most of them are "arithmetic mean range" gathered for 3 consequtive playthroughs and repeatitive playthroughs of some specific moments.
Common elements for tests: Cerberus armor, all ammo powers evolved via squad/30% spare. Tested both versions of 6th step of ammo evolution. Weapons and mods were maximum level available - V, VII or X (mostly).
Proving grounds - Mars, Lesuss, Gellix, N7 missions. Various story mission, if situation allowed (Eden Prime, Citadel coup).
I tested all ammo variants, brushed off CA and DA, because it either I royally sucks with them, or their raw damage sucks. Could be both, of course.
Sorry, if I offended your feelings.
Banshee.
With stripped barrier - armour only.
IA.
Widow - 7-8.
BW - 10-12.
Valiant - 15-17.
Indra - 85-90.
AP.
Widow - 8-9.
BW - 12-15.
Valiant - 15-20.
Indra - 125-150.
WA.
Widow - 9-10.
BW - 11-14.
Valiant - 18-21.
Indra - 180-200.
Regarding OSOK.
Without full HK armor set and/or Barrel+Concetration mod, W, BW (HK required), J do not deliver OSOK on body or extremities shots on CAT or Cannibal, so headshot required.
Valiant with Cerberus armour but without Barrel+Concetration mod do not deliver OSOK even with headshots for Cannibals, but do that for CAT and, in case of slot shot - Guardian. Two headshots from Valiant are enough to dispose of Engineer or Centurion.
Gathering and sorting out additional info on other types of enemies now, if someone is interested, I will list it later.
General info on Indra.
AP/IA (any of 6th step variants) - generally destroy Atlas' armour in 4 mags (each mag strips 3 bars of armour or so.
CAT is killed via 5 headshots or ~10 body shots.
Centurion, Guardian (with AP ammo or mod), Engineer, Marauder - with 1 mag of bodyshots. Guardian could be killed via 10 more or less careful slot shots.
Cannibal - 7-8 careful "near headshots" or 11-13 of body shots (generally 2 Cannibals from 1 mag).
Ravager - 1.5-2 mags.
Nemesis and Phantom, under Stasis wiped out via 2 mags, or with Overload/Energy drain with one mag (IAE preferable for AoE and generally for "afterburn"), but really - GPS makes bringing knives to gunfight obsolete much better.
Calculating proper number for Brutes and Harvesters a little bit more difficult due presense of teammates there, but generally 4-5 mags are enough.
On Menae, in Victus camp, I was able to destroy brutes before they crossed place where Blackstar located. No other rifle, even Valiant allowed me to do that. Well, maybe I royally suck with other guns, can be.

Numbers on Geth are under processing now, but generally they lie within ranges stated above.