If you are going to try I would suggest using epoxy to cast the gun. Far easier to work with. Sand casting aluminum is not horribly difficult, but is outside the reach of most craftspeople. One thing to also consider is that aluminum has a shrinkage factor, so you will need to make your pattern oversize and allow it to shrink to size as it cools.
So, as an at home alternative. Take a plastic soda bottle, cut it so that you have a flat piece of plastic, warm in the oven till soft and pliable. Lay it over a handgun, put it inside a vacuum packer and suck it down. You now have a perfect form for the gun for free. Pour the mold full of epoxy, perfect copy. Best to do it on each side to get a true copy. It is a little more complicated than this, but not much.