i am currently using stuff called gelflex, its a silicon like moulding material which you chop up and melt, once liquid you pour it over an object and wait to set, it sets and holds the imprint of the master so you can cast from it.
the best thing is that if you don't want it any more or its damaged you simply cut it up, re melt and mould again.
i am currently making an odst smg and i am using this stuff to make moulds. as long as the material will not melt, eg don't use polystyrene, it will work, i use foam board to make my gun and it works well with that.
i will be getting more of it today, on ebay it costs about £9 excluding postage which is £6 but each other item is only plus £1.50 postage. plus on you first purchase they send you a code that will knock off 10% on your next purchase. so next i will be buying 3 more kg of it which will cost me about £33 which in the long run saves me hundreds of pounds seeing as that weight in silicon could cost me over a hundred
check it out even if just to try it on something small.