First, just to be clear, rules already exist for shaped charges in the Shadowrun rule book under the heading "Placing Explosives" on pages 96 and 97 (American softcover). What will be provided here is a clarification of those rules so that they will be easier to use.
First, what is a shaped charge? A shaped charge is, for lack of a better definition, a formed piece of explosive material that is used for cutting through structural material, usually doors, walls and armor. The shape of the charge is usually made up of a metal frame or container that is designed to direct the force of the blast in a single direction, either in a plane or in a straight line, resulting in a cutting effect. Certain shaped charges can also be made with the explosive material itself. Because the force of the explosion only has to move in one direction instead of in a sphere, all of the compound can be spread thinner so as to maximize the effect of the explosion.
Examples of shaped charges include small hole punchers and entry charges. A small circular charge can easily be made that will cut a hole in a door or also be used to blow though door hinges or the like, knocking the door open. An entry charge can be a light metal rectangular contraption made of angled iron or steel. The explosive compound is placed in the acute angle, the whole device is placed against the surface to cut through (held in place by a pole or an adhesive), like a wall or door, and detonated. The actual force of the explosion is directed so that a section of the surface is cut out, as if it were an article from a newspaper, without seriously harming anything near by. The area affected by both of these types is very localized and damage should only be applied to the barrier in most cases.
How does this work different from grenades? When the explosion of a grenade, or other explosive that hasn't been placed for a specific effect, reaches a wall or barrier, twice the barrier's rating is used to calculate the chunky salsa effect. Anything left over gets through the barrier to some extent. When an explosive is placed, like a shaped charge, the barrier's normal rating is used to resist the damage. The person placing the charge rolls their demolitions skill with a target number of 2 adding any successes to the power of the explosive. If the resulting power of the explosive is equal to or greater than the rating you get a hole that can be moved through. Read the table for exact effects.
Simple, neh? Since the power of the blast was equal to the rating of the door the desired effect was achieved. Here's another example of possible uses of shaped charges:
Why haven't you been using shaped charges in your game? :-)