my honest opinion buy low entry hunting shotgun, however that said ask the manufacturer for the assembly drawing.
the barrel should be removable. can not believe there is no replacement barrels.
I am not a gun smith but I am an engineer, the deal is the thickness & hardness of the barrel is stress for safety.
there must be enough thickness at the end of the barrel to machine a thread. that accepts a thread tube (choke)
My honest opinion go buy a low end pump shotgun, Mossberg 500, or Remington 870 or equivalent.
after the cost you could just by an other shotgun, and it would be safe. Like Steve(ILOVESPRIG) said modified choke for
multipurpose hunting and steel shot, I use a mossy 500, mod. choke, pump. cost me $400 on sale.
I beat the crap out of it and it keeps working.
or buy a nice new higher end shotgun for the future, and make sure it feels good while holding it.
I was shooting a borrowed shotgun a long while back shooting at clays, I could hit poop with it because
it was to big and heavy for me, I have medium build body frame.
with my old winchester model 50 I rarely miss. problem is after a certain year it's not steel shot compatible.
and has full choke which both will blow up the barrel. so it's becoming antique.
most barrels are very easy to remove and replace. my mossy has one screw , to remove then twist the barrel,
has a butter fly and a slot in the carriage , it easily is removalble.