Seb,
What you have is a bit of a collectors item. I own one of these barrels and boy can it be fun. First of all you are correct about it being used for shot shells, But they went one step further and extended the chambe to accept whats called "hot Shot " shot loads. That screw on choke creats a full ckoke pattern at 20 yrds, And yes it will take full power loads. Beware of the recoil though. I ran some full power 300gr. XTP loads threw it and my wrist hurt for a month. If you aquire it send me an email to
mberry1217@juno.com and i'll email you some printed loads from Hot Shot. Also beware that that barrel requires an octagon forestock.
Good luck, mark