Yes, the state, and some city, fire codes...have all kinds of limitations on quantity and storage requirements for powder and primers - both for commercial operations and in residences.
His insurance company may have additional limitations on coverage or exclusions for selling reloading components...or especially "reloaded" ammo... /or they may reject his application or cancel his policy - if they find he is selling reloading components And all commercial insurance policies are subject to audit at the end of the policy year ...and will typically require a new application every year - where he would disclose his sales by category /
and while this is good general info ---every state is a little different on the wording in their commercial insurance policies / regulated by their states insurance commissioners / he should talk to his agent or broker before he adds reloading components ( as well as understand the fire codes ).
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Picking one powder for shotguns and handguns ...would probably be Hodgdon Universal or TiteGroup .../ but neither one of them work too well for rifle.
If he truly does this...he can find 2 best powders for rifle, 2 best for handgun, 2 best for shotgun..../ but guys that are really into loading for shotguns will want at least 4 lb kegs...or better yet, 8 lb kegs. ( 6 feet of shelf space goes away pretty quickly .../ or he just keeps a placard showing what is available...and he keeps the powder in a steel approved storage vault) vs on the shelf.