Here in the UK, we don't have the high temperatures, but we do have plenty of humidity!
After use, a quick clean at a minimum (proper clean can come later).
Quick clean consists of a push through with a bit of tissue (or a patch or piece of 4x2) to remove loose fouling/residue, followed by a few strokes of a Payne Gallwey pattern brush with a few drops of a suitable bore cleaner. I use Youngs 303, though the type is not important but it MUST be a cleaner for gun barrels - as these contain suitable neutralisation for the powder residue and solvents to dissolve stubborn residue.
This can be left overnight to 'work' and either the cleaning finished the next day, or a tissue pushed through and the gun used again.
If you do this, the gun will not rust and will be kept in good condition - and its not a lot of effort.
WD40 is a water displacer - a job it does well, but not a solvent cleaner and never a good lubricant.
