WD-40 stands for Water Displacing (formula 40), and is NOT a gun oil. Damage can be done to your firearm over time if you lubricate it with WD-40. The only time you should ever use it is if your gun gets absolutely drenched. In that case, it is acceptable to use it to spray your gun with to remove any water that may still be on it. It should then be wiped off, and a suitable gun oil used to lubricate and protect your firearm.
Gun cases are great for protecting your firearm when traveling, but not so great for your gun's finish. Gun cases tend to trap moisture which will create rust. Then, you will have to bring it to me for a reblue, so save yourself some time and money and never store your firearm for a long period of time in a gun case.
For those who regularly use their guns, it is important to not only clean it, but to bring it in every so often, usually once a year, for an in-depth cleaning service. For waterfowl hunters, or others who tend to get their firearms wet, an annual service is a must. While you probably field strip and clean your gun regularly, the gunsmith will completely disassemble your firearm and clean and inspect the parts, looking for any safety issues, function problems, worn parts or factory recalled parts. He will also treat porous areas of your wood stock to protect it from oil damage. Speaking of which:
Excessive use of oil, and improper oils
We see a lot of cracked buttstocks in this business. One thing that often causes this is the owner drowning the gun in oil, then standing it up in a gun rack. The oil will find its way into the face area of the stock which is usually porous and unsealed, and soak the wood. Oil soaked wood gets weak, and recoil will crack your stock, so go easy on the oil. Excess oil also tends to trap dust and other things which will gum up the works. Usually a light coat is plenty. Since we are talking about oils, people often ask me which one I use. For a time, I used RemOil, that is, until I saw a test conducted with various lubricants to determine how well they protected against rust. I don't use RemOil any longer... The most common lubricants I use are Break Free CLP (or other CLP's), and a fairly new product called Seal One. Seal One is a dry lubricant and is a very unique lubricant and protectant.
The 12/20 catastrophe
A 12/20 happens more often than you think, and yes, it is catastrophic. This condition happens most often when a shooter gets a .20 gauge shell mixed in with his .12's (like in a hunting vest). At some point, the .20 gets loaded into a .12 gauge, and when the shooter attempts to fire, nothing happens, because the shell has slid down into the bore of the barrel. The shooter opens the gun and assumes he forgot to load it or that the shell did not feed, and he then loads a .12 gauge shell into the chamber. When the gun is fired, first of all, there is an obstruction in the barrel caused by the .20, then when the shot charge strikes the primer of the .20, it explodes. It makes quite a mess of your gun, hands, eyes and nearby shooters. I would avoid that if I were you.
Excessive recoil can be a problem for the shooter. It makes you slower to get back on target for a follow up shot, it can cause flinching, and it is downright uncomfortable. There are various muzzle brakes, portings, recoil pads and recoil reducers which can help. Just ask your gunsmith!
The Shade Tree Gunsmith
We all know that guy, the tinkerer, the guy who can fix (almost) anything. (Hopefully, you aren't that guy.) Well, when it comes to guns, the shade tree gunsmith can be a problem. There is always that guy who will tell you that your trigger pull is too hard then offer to do a "trigger job" for you. Don't do that...You see, there are basic safety rules (as you can imagine) when it comes to the firing system of your gun. If those safety rules are not followed (such as sear engagement and sear angle to mention a couple), the gun will be extremely dangerous and could fire if bumped or even on closing the breech. If you want your trigger pull reduced, or want to reduce creep or overtravel, please bring it to a qualified gunsmith. That is definitely not a job for your neighbor (unless he is a gunsmith...). I personally don't work on the brakes on my car because I don't know how, and it wouldn't be safe, just as you shouldn't let an unqualified person work on your firearm. However, if you do, I would be glad to refix it for you when he's done. As for the hole in your ceiling, you will have to call someone else for that.