I just bought a new Knight .50 cal. I was told that once you load it, pellets & slug, I can't leave it in the barrel more than 48hrs or it will start corroding the insides because of the pellets. Is this true? I'd think you could leave it in longer?

Too, it was recommended I use two 50grn pellets and a 348grn slug, how adjustable is this combo? I'm looking to be effective to 125yds or so.

Thanks / ElkNut1