Don't know if you handload shotshells or not but you will want to convert a 7/8 oz or 1 oz 12 gauge recipe to rocksalt.
I've never weighed rock salt so I'm not sure how much volume 1 oz takes up.

Be damn embarrassing to have a blooper and then have Mr. Wile E. Coyote standing there grinning at you.





For those not familiar, a "blooper" is an underloaded (or defective) shotshell where the entire shot column lazily exits the barrel and falls harmlessly to the ground about 15' (for example) away.
Stop shooting and check for a stuck wad or the next round might grenade the gun.