Milwaukee Caulking Gun Handle Bar Cap C18PCG-400T C18PCG/310C

This is a cap that fits onto the rail bar that pushes through into the caulk chamber, in order to push the caulk out the end of the gun. This lever could be prone to snapping, especially if there is a blockage in your tube. In this case it will be necessary to replace this part, otherwise, you may not be able to pull the bar back out to load in a new tube of caulk.