Powder residue, lead and bullet lubrication can get packed in your barrel's chamber, which can cause feeding and extraction malfunctions. Unfortunately, your .38 or 9mm bore brush is the wrong diameter and usually too soft to remove chamber contamination.
You need a Wilson "Chamber Cleaning Tool". This tough little tool has a sturdy Delrin handle and very stiff stainless bristles to thoroughly clean your chamber. If you own a .38 Super or 9mm handgun you need this little tool. Stainless brush is hard enough to remove stubborn fouling without damaging your barrel.