I personally have shot multiple and witnessed multiple bears shot with brenneke slugs. They are a bad dose. I haven’t seen a bear tote one yet. Feel free to shoot me a pm for more info.
In-stock best prices for Brenneke 12 Gauge Ammo shotgun slug | Cheap Brenneke 12 Gauge Ammunition shotgun slug - AmmoSeek.com
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Seems to be some options. The link I followed for the Green Lightening showed in stock at Clark’s Amory. I’ve never bought from them, but they showed in stock.