For smaller quantities you can use one or two piece RTV rubber mounds which are not too hard to make. Larger quantities would really need a centrifugal casting machine, this not only allows quite a few parts to be cast at once it also produces a better product as the action forces the metal into the mould so less faulty castings.
You may also want to ask on our sister forum "military modeling" as they are more into this type of casting.
Having done quite a bit of figure modeling you may also want to conside using PU resin rather than white metal, I prefer it to work with and you can use more one piece moulds which don't leave the part lines that you get with two piece moulds. If the parts are for the AFV market then PU is more popular.
Either way it takes a lot of time to learn how to place the split, gates and runners to ensure a good casting without flash, badly positioned parting lines and no distortion shrinkage.
J