WD-40 is fine to clean with. Excellent to displace water (that's what its for), and WILL work as a light lubricant, temporarily. For longer term use, Oil is better.
One thing you should NOT DO is use WD-40, or any other "penetrating" solvent or oil on ammunition. Do not lube the gun with a loaded mag in place, or rounds in the cylinder. DO NOT use so much that it may creep into those places after you are done. IT will get into the ammo, and possibly render it inert.
All else being equal (and it almost never is) bigger bullets tend to work better.