The two main types of single shot airsoft grenade launchers, Mounted or Stand Alone.
If you choose a stand alone or mounted airsoft grenade launcher the first decision is what type of shell to load it with. Multiple BB shell, Soft foam ball,or rubber bullet outer sleeve style. Sometimes the playing field will have already made the decision for you and not allow specific types, its common for a field to have rules in place for either the rubber ball or rubber bullet style of shells and you will be limited to the multiple BB shell.
Launchers have an advantage over standard hand thrown grenades, the biggest advantage is you don’t need to put down or sling your rifle to use them. Also no fumbling around in your pocket, they are as ready as they are going to get making it simpler and faster to launch grenades compared to throwing them by hand.
Apart from the kinds of airsoft grenade launchers shells you can choose,
there are also the types of launchers: short and long outer sleeve. A long outer sleeve is able to launch grenade shells further but can be cumbersome and heavier than a short style. Even if a short outer sleeve doesn’t have the range of the long outer sleeve, it gives you more maneuverability and can
sometimes be reloaded faster. Some launchers may not be compatible with your style of rifle or and additional kit may be required to connect the two. Confirm before purchase.
Mounted launchers have an advantage over stand alone. Where your rifle points your launcher points.
Single shot airsoft standalone launchers range from the King Arms M79 launcher that mimics the Vietnam weapon of that time, to more modern versions like the ASG GLO7. Stand alone launchers are best used with a team. When one member is switching to use the launcher one is covering, it can also be moved around the team member that is in the best location to use it.
Finally there are kits that will turn your mounted launcher into a stand alone system. The best of both worlds.