With compressed air and the co2 you need away to catch control the ball as it will have some force. think air rifle here.... The grease gun works well and is a slower applied pressure as the grease gun adds pressure then it equalizes and levels with each pump. A high pressure grease gun can get to 2500 lbs pressure easily. It is a mess to clean up after and will be a job to get all that grease back out of the barrel. Im wondering if a grease gun dedicated too this and using vegetable oil might do the job, cheaper and easier to clean up after. The big thing is finding the right thread in a zerk. Ir making an "adapter to get the right threads.
THe problem with the ball wirms is they expand the ball when screwed in wedging them in tighter. a piloted drill the size of the shank in the worms thread to drill a pilot hole helps relieve this and makes pulling the ball easier. I made one for my 50 cal renegade. pilot is 1" long drill is 3/4" long I use this ona t handled cleaning rod to drill a hole threw the ball then put the worm on and they pull a lot easier , almost ae easy as they pushed in