A 5 gallon pail fit inside of this, the spring loaded grease follower fits inside the 5 gallon pail of grease. You DO NOT have to fill the steel tank with grease. It pumps vey well and all I had to do was get a regulator, change the tip end of the gun handle, add air coupler and 5 gallon pail of moly grease. Primed quickly, pumps like crazy.I'm using it in my business and on my equipment. Good bang for the buck !