I grew up hunting deer with slugs in upstate New York. That was before rifled barrels, so we all used smoothbores. Foster slugs work fine on deer, almost always exiting out to 100 yards or so. However, Brennekkes were always a much harder slug, and always exited even on the old THS. In fact, my dad always thought I was crazy for not shooting through brush and trees with Foster slugs, as his Brennekkes would cut through an awful lot of it and still get the job done (I'm sure the naysayers will chime in here, but hey, I saw it done a number of times with my own eyes). I also know that Fosters will not penetrate the skull or (fatally) the neck of a Hereford bull at 50 yards. Brennekkes might have but I'll never know!
Brennekkes sometimes shoot well through a smoothbore, and sometimes not. Ithaca Deerslayers were very popular where I grew up, mainly I think because they had fairly tight consistent bores that shot most any slug accurately enough. I still own mine and heartily recommend them. They generally would outshoot Rem 870s of that period.
Accuracy wise, if you cannot hit a deer in the vitals out to 100 yards with a smoothbore, it's usually not the gun's fault! They do not shoot like a rifle, but who cares?! For the record, I once shot a wounded deer broadside with a Foster slug at 250 paces. The slug exited through the deer's lungs and he died quickly.
