Hi Robert!
Coming along nicely.
Surface lines aren't visible in the picture, but I'm assuming that the side of your gun is all one surface, correct?
If so, the easiest way to add the vent slots is to use the boolean tool. Using the polygon tool, make a quadralateral the shape you want the slot. Next extrude the quadralateral so that you have a box that extends all the way through the gun, sticking out slightly from both sides. Switch to object mode, then select the gun follow by the box. From the Object menu, select Boolean > Subtract. This will make a hole in your gun the shape of the object that was cutting through it.
Sometimes, you may need to do some minor clean-up after using the boolean tool. If you have faces you don't want but the vertices are correct, then select the face and choose Surface > Remove Surface Only. Conversely, if you need to add a face, select the vertices in counter-clockwise order and select Vertex > Create Ordered Surface.