Smells nice but not that good as a rust preventative. I learned the hard way. I use wonder lube and it seems to do what I need both prevent rust and lube for shooting. Just one of many options though. Good Luck!!
WD-40 is not a rust preventer.It is a water displacer.Guy on another site did a test with multiple nails that were all cleaned and all oil removed.Nails were then lubed with numerous oils from gun oils to transmission fluids.Board was placed outside exposed to all weather conditions.Last I heared olive oil was best and gun oils were failing.
During the muzzy season my barrels have NO oil of any kind in the barrel till after the season.
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I have used BB in my ML's for years and I have never had a hangfire because I didn't clean it from the barrel before shooting it.I use a very thin coat of BB in all my muzzleloader barrels.I would rather have BB in the tube than a puddle of WD40.JMO