What would happen if you threw a tennis ball full of buckshot into the center of an MRI cylinder?
Would it explode?

I think buckshot is lead.

Probably need to use steel ball bearings to achieve what you are after.

AND - even then, it won't be pulled apart evenly like an explosion --- it would likely just veer off to one side of the MRI magnetic field as a unit (the tennis ball).

Even if the buckshot is non-magnetic, it is electrically conductive and will still be subject to effects from eddy currents.

I would expect resistance to movement normal to the magnetic field. This is evident in a non-magnetic aluminum block in MRI MAGIC

-Charles

I believe watrefowl loads contain steel shot. I'm not a waterfowl hunter but I think it is required to use steel instead of lead.

you are correct. it is illegal to use lead shot in some places, steel ,tungsten, and anothre i cannot recall at the moment are required for water fowl hunting.