I bought to of these mags for my Echo 1 Stag-15. They fit but do not lock in place, so in order for them to stay I have to hold in front of the mag while shooting. Which isn't a big problem because I usually do that anyways.
-120rds for a mid-cap is great
-feels very sturdy nothing loose
-little bit pricey
-does not lock in place for Echo 1 Stag-15