Screw it firmly on to an old battery. Then clamp the battery in a vise. Make certain that the end of the battery tube with the fitting inside is included in the clamp. This will prevent the battery tube from collapsing. Then wiggle the carto around in various ways to stretch the metal of the tube. Once it begins to move around a wee bit, continue while pulling. Don't get in a big hurry and try to keep the stretched metal to a minimum. If successful, it will not leak when it is pressed back on. If the stretch is too much, simply swap ends with the tube. Use the vise to press it back on. Make certain that the fitting is exactly square with the tube while pressing. Otherwise, it will stretch the tube.
Mine always leak whichever side I used. Any ill effect if a bit of silicon is applied to it before pressing it back on?
edit: br, those not soak in vinegar is fine the way I dry burn them