Yonks ago I bought two 40 rd mags. They weren't much bigger than the 30 rd ones and still fitted in my webbing nicely. However, the monkey's confiscated them on my way back from Kosovo, thinking they were AK mags. cnuts.
They work very well, like everything Surefire produces they are well engineered.
However, unless you are pretending to be an LMG the 100 is overkill unless it is carried in a vehicle as a 'just in case' magazine. The 60 you can get away with on foot just about ............ admitedly it was a demo session but I got bored firing 60 rounds out of an M4, it was getting to the stage of, "will this thing never empty".
Before anyone gets all excited, no its not me its one of the Surefire reps.