In reply to jellyknees:
The Belaymaster is quite flat and a bit narrower across the top then some belay carabiners. It is designed to work with a Single rope controller (they often comes as a set). The 'gate' must be used with a SRC as if the device slips around to the narrow end you have a pulley, not a belay device!
Naturally, the BM can be used with other belay devices. However it is worth checking to see how well the device 'sits' on the carabiner. With a standard ATC, due to the underside being wide and quite deep, there is a tendency for the belay device to slip sideways and lodge itself on the corner of a Belaymaster carabiner, gate or no gate. Not very good at all and a standard HMS works much better. However, with the new 'v groove' ATC XP the Belaymaster carabiner actually holds the belay device is a more central position and it is much less likely to slip around the carabiner then when using a standard curved one. I would think a Belaymaster would also suit a Variable Controller quite well.
The best advice is probabaly to try find what shape of carabiner your belay device of choice 'sits' on best. If it sits well it is unlikely to slip round in any case.
I use a Belaymaster with an ATC XP but have dumped the plastic gate as it just gets in the way of the wire.
Hope that all makes sense!
Regards,
Howard.