The spring is strong and it does expand to 45 rounds. However, if you slap the bottom of the magazine when it is extended, it becomes a 30 round magazine, so you have to insert your magazine from the neck rather than from the bottom.
I have a Rock River Arms LAR-15 and these magazines don't quite work. I had to cut the plastic molding on the back. The original molding is narrow, rounded where the bolt extracts a round from the magazine. My NHMTG magazine has ise wide and squared. I took an exact-o knife and cut the plastic into a wide, square shape and it seems to work well. Still having trouble getting the magazine to fit into the weapon with a closed bolt. I can't slap the magazine, otherwise it collapses to the 30 round mode.
Other than that, these magazines are built to last!