If my friend and I wanted to see who was better with the M4A1S in a 1v1, how could we 1v1 using only the M4A1S or some other guns (on Dust 2 or any map)?
Firstly, start a multiplayer lobby and start a casual game - making sure it is private of course - then start the game. Go to the pause menu and vote to change the map to whatever map you desire: it should instantly change the map. Now invite your friend(s) from the pause menu or from the Steam overlay.
There are many commands to practice by yourself or with your friends in CS:GO. Most of these commands are listed here: http://www.tobyscs.com/csgo-practice-config/. For a 1v1 with a certain weapon you would kick the bots, increase the starting money, possibly enable show impacts or whatever you want really.