If you have long mags (you know them 450 rounds high caps) then you are good to go, but if you are using regular 300-350 rounds high caps, they would sit in the kangaroo pouch too deep and will stick out very little, meaning that it will be hard for you to take them out. What I have done is- I got a cardboard tube of a aluminium foil roll and cut that tube in a right size so it fits in the kangaroo pouch, now my mags stick out more, therefore it is easier to take them out!
If you are going to get a Krytac make sure the mags fit (ask people on internet, read posts etc.) them m16 Vietnam era mags don't seem to match the Krytac CRB as the gun is more of a CQB/quick response type of gun, but the mags are more of a DMR style.