Hey Ram,
I would suggest you get to that shop and speak to somebody regarding what you have been discussing and the responses. For instance, designing a handgaurd may not be too difficult, but a 2d sketch will convey the basics (ribs, dimensions/spacing) but if you are going for something more unique than the 10 trillion that already exist, just holding sights on the side and the m240 where the scope goes ( :o ), it may be hard to convey that intent in a 2d drawing.

With that said, Luis suggested ViaCad which I would also agreee with. Huge bang for the buck. You would only go with BobCad in the instance that you planned on moving to doing the cnc yourself. (And even then, some of these others are more powerful "CAD" applications)

Anyway, good luck with it.