I have several of these mags, and i love them, As for the circle thing?
It lets you disassemble the mag. If you want, you can remove it, and inside you will find a spring, with a plastic spacer in it. If you remove the spacer, you will bump the capacity of the mags up to ~130ish.
Its a pain at first, but after 2 or 3, you get the hang of it.
It's better to light a candle, than to curse the darkness