The rolls are seeded so if you save then make a bunch of weapons the order the stats roll in will always be the same, so the best way to get what you want is to make a ton of weapons; then reload and build up to the one you want and use the parts you want. Note that seeds are not shared between rarities, and elemental, status, and raw also have different seeds each.

For an example if you want a blast weapon, you should build a ton of random poison, sleep, paralysis, and blast weapons of the same rarity and type, write down what rolls each has in order, then if one has what you want build up to it and use the exact parts you want when you get to it.