I have had two memorable bad experiences with WD40.
I also bought a gallon and I still have some left.
I lubed an Op Rod with it. It ate the op rod and my rifle had to have extensive repairs.
I stored many steel reloading items in WD40 including a Forster Trimmer. Enough WD40 that it was wet in the Ammo Can. When I removed it all from storage, the Forster trimmer had a lot of aluminum eaten away. The spinning handle really looked terrible.
Never Again!