The greatest use of explosives is to take out a guy who hides behind a corner. You see them, they see you, they duck for cover, and BOOM, you get them with an AT-4. It tells a player that hiding is no good. Guage the effective blast radius by going into a private server with someone, and fire close, to see where they go down, and then fire succesively further away. You will need to shoot each other after each try, as usually the damage is accumulative, and you want to see what an explosion does to someone at full health. The other thing to test is proximity to you. The ultimate goal is to be able to blast a satchell on someone, or launch an RPG, get a score on them, and see yourself go to minimum health. They will be wondering how you ever survived it, when they were so close to you. The trick is to learn the minimum range, then fire an explosive just behind them. They are inside the effective area, and you will be 1 foot outside of it.
Grenade launchers are the single most effective weapon in a game. They will vary for trajectories and power, so make sure to do plenty of testing with a grenade launcher. Practice with it until you can hit a target at 100-300 yards with 80% accuracy or better. See how the projectile flies, and make sure that the game has it programmed for a realistic flight path.