Hah.
It's actually not a big deal either way. As long as the contact points are lubed.
two questions...
1. what type of gun is it? (Doesn't really matter though)
2. Where do you live? (also doesn't matter much, I'd treat it about the same unless conditions are fairly humid.)
So basically, you want a light film of oil on all metal parts, in most peoples opinion that means ALL METAL PARTS, it doesn't matter if they are coated with some kind of finish, although some people might make exceptions for painted parts or whatever. Generally speaking something like a Glock with whatever type of coating it has over the slide inside and out, and other coatings like nickel plating, ceramic coatings, etc. etc. etc. blah blah blah, will all be fine with a light coat of oil on them inside and out. Although they may not NEED a light coat of oil, (however it may help with rust prevention in humid and or salty conditions) it will add just that much more protection, and it will also aid in cleaning the gun by providing a bit of a barrier between the grime you encounter from firing the gun and the finish it's self. Again, this goes for inside and outside the gun.
Things that need no oil are wood in general (Unless the wood has been finished with oil and needs occasional attention) plastic, rubber, etc. If you have bare steel, stainless steel, blued steel, metal of any type really, it's best to have some oil on it, and then wipe it off to leave a thin coat behind. Pretty much all guns in my experience work perfectly fine with just enough oil to keep the contact points lubed, and then everything else wiped down.
This is the same in my opinion with magazines and their springs. The inside and outside of magazines get oil from me if they are metal. I might attempt to wipe the inside dry a couple times after lightly oiling since it actually holds the cartridges and people always talk about the idea that too much lube can attract dirt and muddy things up, and can also gum things up in extremely cold conditions, but generally I just "wipe on wipe off".
Once you've done it enough, you'll get the idea. It's pretty simple. There is no need to leave a lot of lube on the metal parts that are NOT contact points unless you are in humid conditions and/or you intend to store the gun for a long time. But it wont hurt anything if you do leave a bit on all metal parts, and generally speaking it's probably best.
As far as inside the barrel. It's always the same... send your last oily patch through, then one dry, and that will leave enough behind so that it's safe to store, and ready to fire next time out... unless you intend to keep it sitting around for years and years or you're in humid conditions then I'd run an oily patch through and leave it that way without following up with a dry patch. Then when it comes time to shoot the gun (years later) I'd run another oily patch through followed by a dry one just to be sure things are clean and clear. But if I had a gun sitting for a really long time, I'd probably give it a once over (quick disassemble and lube) before shooting it anyway, just because.
I hope that helps...
That's what most anyone who's done a lot of shooting or has lots of experience with guns in general will tell you... unless you run into someone who just really likes to run their guns dry, or doesn't like to spend much time cleaning.
One last thing...
This applies to all guns, not just semi auto's.
