I've been using WD-40 for years... but the only place I ever used it on my guns was for a quick spray and wipe of dirty or wet shotgun bores.
For cleaning an action or trigger assembly, there are much better products than WD-40. Same thing goes for lightly lubricating and protection afterward. There are plenty of good gun oils available, and the amount needed is so small that the cost per gun per year is negligible.
WD-40 absolutely will leave a waxy buildup that can make things sticky over time. It may have more of a tendency to do this than other lubes. But the fact is, any oil or grease will dry out or oxidize over time. They will all thicken and collect dust and dirt much easier than a perfectly clean surface too. The old dried out dirty stuff should be removed before just adding more. In sub-zero temperatures, even a small excess of lube will thicken and create the drag that can slow down a firing pin.
The military has for years used firearm maintenance products with the abbreviation CLP. They are all in one products that are used in the field to Clean-Lubricate-and Protect. They are a compromise. CLP products won't clean and dissolve old grunge and dried lube as well as something like Brake Cleaner, Carb and Choke Cleaner, or Shooter's Choice. But they also won't leave parts so clean and dry that they quickly flash rust either. It shouldn't take a lot of intelligence to know that these stronger solvents could damage a stock finish, which is why guns are stripped instead of indiscriminately spraying them on everything.
So the problem isn't so much with WD-40. The real problem is people who use it incorrectly, and in places where there are better products to use.
