Have you tried Gunstruction yet? If not, prepare to lose a few hours of your Saturday.
Gunstruction is a browser based, 3 dimensional, virtual AR-15 building tool. It lets you choose from an ever growing selection of real AR-15 parts to get an idea of what your build will look like and how much it will cost.
It is a pretty robust tool. You choose your parts all the way down to things like trigger pins and barrel nuts. It is smart enough that your choice in barrel nut will effect your hand guard options, your choice in barrel will effect your gas block options, and various other similar relationships are built in.
The list of parts is pretty extensive. There are parts from Larue Tactical, Magpul, Daniel Defense, Elzetta, Trijicon, and more. You can even apply various DuraCoat finishes.
I would love to see them add the weights of all the parts so you can get a rough idea of the potential weight of your dream gun.
This is a useful tool and it is an absolute blast to play with. Check the AR15.com Gunstruction Beta site.