Inspect the end cap to see if it is broken or if it has simply popped out of the bat. Bat end caps have ridges that grip the inside of the bat barrel and hold it in place; if these ridges are chipped or broken, you cannot reuse the end cap, and you will need to buy a new one.
Apply a thin layer of urethane epoxy inside the barrel grooves. If necessary, use a paintbrush to spread the epoxy evenly. Do not get epoxy on the outside of the barrel or on the end cap.
Insert the end cap. The end cap ridges snap into the grooves inside the barrel, ensuring a tight fit. If you have difficulty getting the end cap into the bat, use dishwashing liquid to lubricate the opening of the barrel and carefully knock the end cap into place with the palm of your hand.
Check the fit. Once the end cap is in place, wipe away any excess dish soap or epoxy from the outside of the bat.
Let the epoxy set. Leave the softball bat in an upright position for 24 hours, or for the amount of time listed on the epoxy packaging.