I realise this is a fairly old thread but I'd like to add my findings for those trying to decide on mags.
I recently returned from a full days play using a couple of King Arms 360rd Mags with my Masada, apart from the aformentioned issue with the mags not engaging the latch easily they fed faultlessly and I didn't have any issues throughout the day. I will most likely be modifying my mags to fit a little easier.
I also tried out an ICS 470rd mag, fits fairly snugly as previously mentioned and isn't a whole lot bigger than the King Arms mags; also had no feeding issues using that for a
