a gun is pretty complex for a beginners model. check out my beginners course to familiarise yourself with the modelling tools.
but having shamelessly self promoted myself lets try and answer the question.
yes you need extra vertices.
i hope you are using a reference image inside blender otherwise this may all sound like nonsense!
add a loopcut to the cube, you need to be in edit mode to do this. press ctrl R (cmd R on mac) and you get a pink line around the mesh press left mouse to confirm the selection now you can slide the loop along the cube once it is position at the place you need the indent press right mouse to confirm the position. do that for each edge of the indent. now select the entie face of the indent and press E you can now extrude that face inward to create the chamber. additional loopcuts and moving of vertices will be required to get the correct shape of the chamber, but that is the basics of it