Oil the barrel.
Wipe it out with a clean dry patch on a properly-fitted jag.
There.
You're good to go, if you're carrying the pistol for CCW or duty use.
There's enough oil left on metal surfaces using this approach to temporarily inhibit rust. If you live in a very humid climate, do the above drill daily. If you live in a dry climate, once a week may suffice. (This is assuming the gun isn't fired in the meantime, of course.)
DON'T leave a visible, thick film of oil on inside of the barrel. At best, you'll degrade accuracy, at least for the first shot. At worst, you may have excessive chamber pressures develop.
Of course, if your gun isn't carried, and the barrel can be wiped out prior to use at the range, you can leave as much oil in it as you please! Best.