It's a modified. I shoot slugs in my full choke 1897 and that has softer metal for a barrel. Foster slugs can be fired in all choke sizes. They are designed to be able to do that.
Been shooting slugs for decades in both my fathers A5 now my son's Circa 1925 and mine Circa 1971 both having full chokes with 0 problems I did calibrate the choke on my A5 recently and after shooting countless slugs it show no sign of wear. I also find it surprisingly accurate out to over 50 yds. without a rear sight. Where I hunt I do not have an opportunity to shoot much beyond 50 yds. so I never really tried to zero in any further distance.
