Peters used to load 28ga slugs many years ago but they're collectors items now.
Slug loads can be found in .410, 20, 16, 12, and I think, 10gauge. None of the factories currently load a 28ga slug. There was talk of a round ball load but I hevn't seen it.
I have a 28ga and would like to have a box of slug loads too.

Bore size: 28ga is .550 nominal

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