Are there any restrictions to what I can load in a smoothbore musket with an SGC?

I've been doing clay pigeon sessions with a view to get an SGC with the club's recommendation, and I was talking with my instructor about whether shotgun slugs were legal, to which he said they'd require a Section 1 FAC, I then got to talking about how I'd be interested in black powder shooting at the local range and asked whether there was any restriction on what to load into a smoothbore musket, to which he said musket balls would also require a Section 1.

Is this correct? Given that ownership of black powder requires a separate explosives license, how is it actually policed what you load in to a smoothbore, given that they're just lead balls which you can cast at home? Will I have to exclusively load shotgun shot into a smoothbore, or is he mistaken and I can actually fire musket balls? If musket balls are allowed, would there be any restriction on something like Minie rounds?