Alternate reload
From DoomRL Wiki
Some weapons have alternate reload modes. Generally, alternate reload modes really do involve reloading the weapon, but sometimes they are used for miscellaneous activities.
Full reload
Full reload will reload the weapon's entire magazine. The amount of time taken is 0.2s plus the time it would take to normally reload the amount of missing ammo. For the combat shotgun, the weapon will also be pumped.
Single reload
Single reload will reload only a single ammo into the weapon's magazine. This time taken is the same as a normal reload.