I would recomend the CA P90. Mainly because ive seen and used the CA P90 and have almost nothing but good things to say about it. Ive only used it once but when I did I wish I had owned one. Its accurate for the barrel size, its small and compact for CQB and pretty good in feild, it dosen't shoot as hard as I would like it to but it is a CQB gun and its not ment to shoot hard. Over all I would buy one, I have no complaints about it.
If your looking for high caps your best bet would be the TM high cap mag that seems to be the best working high cap mag. Or if your looking for a really big magazine look into the box mags they hold around 1,500-2,000 rounds pretty much a compact SAW.