In Rifts© you are reminded of the need for S.D.C. weaponry in an M.D.C. environment constantly. The need is there, and the facts are true you can not hunt with a mega-damage weapon, you often can not carry them into cities, and shoot-outs with them do an awful lot of collateral damage to non M.D.C. structures. The reasons not to use them are almost as endless as the reasons to use them. So, in steps the S.D.C. weapon, however there aren't any currently being manufactured!  The creators of Rifts have so far left out modern S.D.C. weaponry from their wonderful collection of books. Instead they suggest using such weapons as AK-47's, and Mac-10's. Weapons that have been out of production for well over three hundred years! So I introduced a few current manufacturers, and suppliers of S.D.C. weapons for my own campaigns. Included here are two of these companies, you can either use them as is, or take the idea and create some companies of your own. Yet another, and a bit easier way of introducing modern S.D.C. weapons into your campaign is to use an existing company such as Northern Gun, or Triax and just add in a branch that manufactures the new weapons that you design.
As far as designing the weapons themselves, I suggest what the game suggests; go to one of their other books (i.e. Compendium of Contemporary Weapons©, or Ninjas and Superspies©) and use the weapons found there as a guide for your own.