For you guys using WD40 in shotgun bores, are you using the aerosol cans?
The reason being that, if so, it might be the volatile propellants helping to soften the wadding.
However, I've gotten away from the aerosol version. I got a gallon that will do everything you want WD40 to do better than the aerosol, depending on your choice of applicators, for the price of a pair of cans.
FWIW, I've found it great to break things loose (it's a great cosmoline cleaner) and get things moving (not as well as proper penetrating oils, but you don't have to be as careful where it goes), just not to keep them clean and moving.
And it actually works great as a rust preventative, provided it's used externally and on parts not under handling. A mill and lathe in an unconditioned Florida garage count as my proof, and after years the unsealed iron only had patina where I bled on them and didn't clean it for a while.
Sure there are better, longer-lasting products that don't hold dust. They also cost ten times as much.
I also won't lube a gun with WD. I did use my leftover 10-weight machine oil, though. Worked better than most gun oils.