Finding a good mag without wasting ammo

[Unbreakable]

Ok so I have a few boxes of AR mags in my safe. I have found that brand spankin new mags are a little tight and don't like to feed the 300 whisper all that well.... Especially chambering the first round. I really don't want to burn up 300 rounds testing 30 or so mags to see which feed the best. Any one got any hints on picking a good mag... The older mags that have seen some use and are well broken in seem to work the best... How about Black or Green followers. The Black have a little more play in them and may better accomodate the chunkier round. Although with that extra play they may not feed very well... Any and all advice would be appreciated. Once I get about 10 mags identified as good 300 whisper mags... I'm gonna mark them and use them for nothing else.