Try to clean it up a bit, acid, naval jelly, whatever. Look to see if
there's a damascus pattern. Some very old shotgun barrels were made by
twisting a ribbon of steel around a mandrel and welding it together to
form a tube. OK for low pressure black powder, but a high pressure
smokeless load could rupture the barrel.

I agree with Roy that it may be the result of a plugged barrel. Did you
find any teeth or bits of skull in the vicinty? 8*)

Cheers!
Red