Use this tip to make your own custom grout gun.
We are redoing our backsplash and using travertine tiles. Because of the finish, we did not want to fill all the contours in the tiles with grout. To avoid this, we started with a plastic caulk tube. We checked on the grout width, which was 3/16 in., and cut the nozzle to match that dimension. Then we got a 3/16-in.-dia. metal rod and inserted it through the nozzle tip to remove the circular plug at the bottom of the caulk tube. Once the tube was cleaned, we loaded the grout into the tube and reinstalled the plug, making our own custom grout gun.
— Neil Rokos, Cochrane, Alta., Canada