The air bleeder on my grease gun could be unscrewed, and if the adapter on another grease gun was unscrewed, then screwed into my gun, I could fill up my gun.
That was very convenient at one place I used to work, because the bulk grease machine was a pain to use. It was a 30 gallon tub with an air powered grease pump on top, too bad the hose was very short, and one of the wheels was broken off.