Long skinny parts are a real pain to cast, but it can be done. I'd do each rifle in its own seperate mold. I use Lego's for a mold box. I think I'd probably start off by pouring about 1/4" of silicone into the mold box, then floating the rifle on its side on top of the silicone, push it down just a little so about 1/2 or less of the rifle is covered, try to make sure that all projections (pistol grip, magazine etc) is settled into the silicone but not covered on top.
Once the silicone cures (I'd let it sit at least over night) smear a thin coat of vaseline (just enough to make it greasy but not clumped) on the cured silicone but leave about 1/4" free of vaseline on one of the long sides, probably the side closest to the top of the rifle (less detail to have to worry about getting free of the mold). Then pour another 1/4" deep layer of silicone over the first. Let it cure (again at least over night). When its done you should have a mold that opens like a book, it will be connected on one side. To use it pour resin the length of the rifle mold, close the mold and knead it to help force resin into the cavity and air bubbles out. Let it fully cure (15 minutes should be good) then see how it worked.
You can heat the mold in an oven at low heat (low or 200 degrees) which will make the resin cure faster, do this before pouring the resin to make the mold warm, don't do it with the resin in the mold. This will also stink up the house so you have been warned. Chilling the mold in the refrigerator or freezer should give you a slower curing time but I have heard it can cause some weird problems with the resin.
Good luck, you may find you like casting parts or double your efforts to find the kits with FN-FALs in them.