Mech mags hover around +/- 2 lbs. At least the RPG's do
Efficiency depends on too many external factors like barrel porting, barrel length, paint-to-barrel match, making it hard to give you a hard 'n fast number.
More porting= more inefficient. Barrels typically longer than 12" are more inefficient, and a bad paint-to-barrel match can attribute to inefficiency.
To get an idea though...
All blow-forward designs get similar efficiency. If you know what an Ion gets, or a Freestyle, youll have a general idea of what a mag gets.