An Omni needs variants... so unless you remove a half ton somewhere, it's not an Omni.
So remove Half-ton of Armor gets you:
1/ 2
1/1 2/2
1/ 2
And a ER Small
or a Pintle Mount which would allow up to a Half-ton weapon 180 degree arch, say Generic Auto-Rifle, Vehicle Grenade Launcher, maybe a Rocket Launcher/10?, Light Probe or even a Light TAG.
Thing is while the Light and Standard clan MG weigh in at .25 tons, the ammo is allowed in full HALF-ton ammo counts. Unless, like it has been stated before, agreed upon before hand, a Quarter-ton would mean 50 salvos... also that Pintle Mount allows you to use ANY Infantry weapon too!
TT