WD-40 is good for only one thing. If you get adhesive on your knife blade from cutting packaging tape, the WD-40 will remove it nicely. As a lubricant, it is very poor. As noted above, it is also prone to gumming.
As far as primer inactivation goes, only a foolish person would apply WD-40 to the cartridge head of a loaded round. However, a gunsmith, Geoff Benze, has been doing a study for over a year with (loose) primers soaking in WD-40 and others soaking in water.
The WD-40 soaked primers worked perfectly well after one year's submersion, immediately after being removed from the liquid, and after air drying.
The water soaked primers wouldn't work when wet. After they were air dried, however, they worked perfectly.
Once again, I am NOT advocating putting WD-40 anywhere near your primers, but the problem may not be as bad as some people think.
There are much better products on the market now for rust prevention and lubrication. One of the best, and most convenient, is Sentry's Tuf-Cloth; this will protect a finish extremely well. The compound in the cloth bonds to the metal, and is completely non tacky in a few seconds. The cloth comes in a heavy plastic zip lock bag, and you just take it out, wipe down your gun, and return the cloth to its' bag.
Sentry also makes a bore protectant, which allegedly smooths the bbl. and minimizes or prevents leading. This is Smooth-Kote. Tuf-Glide, the active ingredient in Tuf-Cloth, is also available in an applicator bottle, and is a terrific lubricant, also 'sticking' to the surface of the firearm. One more product is BP-2000, which is a powder you apply to critical areas like trigger contact surfaces and hammer-sear interfaces.
You can get an armorer's kit, which contains all of the above products for about $30 from Discount Knives :
Just for the record, I have nothing to do with Discount Knives or Sentry Products, other than being a satisfied customer.
I owe the pristine bluing of my 1957 GC to a Tuf-Cloth. I was shooting with a number of people in AZ last year, and discovered to my dismay that no one had any cleaning equipment to loan me; they were all shooting Glocks. Fortunately, one had a sample size Tuf-Cloth, and that saved the bluing on my GC. Hope this helps, Walt