For the F2000 you have to, although this is probably well known, make a indent in the magazine to allow it to release. Hi-caps do not have this problem as the feed window/trap is already pressed in slightly.

Otherwise the F2000 would be my choice, unique, and bullpup :)

See - that's just it. I'm not a fan of using hi-caps or modding my mags.
I really do like the Masada - or perhaps a custom build AEG?