Metal mags tend to me expensive and, if you're planning on running low-caps, then you will want several of them.
Moreover, you will want to be dropping them on the ground for faster reloads...typically, this isn't great for mags and if your mags were expensive, you might want to take better care of them.
My recommendation would be to get STAR plastic low-cap mags. They come in at about $5 a mag and come in lots of capacities and colours.
In terms of BBs, get BB Bastard and never look back. He's a Canadian retailer, member of the ASC community and a great guy. His product is top of the line and well priced.
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