In some mods there are Under barrel attachments that can be used and I have a mac (if it effects anything) and i cant figure out how to use them. thanks for the help

That entirely depends on the mods. There is no standard alt fire button. Some mods use the pie menu, some use special keys, some use standard key combinations, and maybe some use AI modes.

Read the respective mods' threads, and if you don't get your answer there, ask in those threads.

Thanks