After many many years of wanting a Browning Auto 5 Magnum, I was able to pick up a beautiful Belgium example at a gun auction. Since this was something that I always wanted for duck hunting, I guess i didn't do my homework first and found out too late that steel shot shouldnt be fired in my new duck gun. I was lucky enough to find a "like new" 32" Magnum barrel from a Japanese Auto 5. I am hoping that it is an easy swap, since I am going on a duck hunt on Sunday.
I just don't want to go the bismuth route.

What kind of barrel did you buy?
Invector screw in chokes or conventional
choke. If its conventional, in a 32" barrel
It will be full unless its been reamed out. Not
good for steel shot. If Invector, pick your poison.
A full choke will have a single star on the barrel.
Japan barrels are also thicker, you will need to sand
some clearance on the forearm. If its first year, 1958,
Change out the double spring for the heavier single spring.
Use the heavy load set up first, then go from there.
On my 1958 i bought a plastic forearm because the heavier
single spring is bigger OD than the double spring.
Get you a 16, then you really will get hooked.

I would call Browning directly and ask them. They are very helpful and you will get the right answer, as they certainly know what and what cannot be swapped out. Best to ask the actual source in the end in order to be totally safe.

Though it won't help you for Sunday one option is to send the barrel to Briley and have them put in screw in chokes while they are at it you might have them lengthen the forcing cones....
I did this with a Win mod 12 heavy duck which came with a extra full choked barrel. While some might find this heresy it has made the gun very versatile and it is doing what I wanted it to do...
Good luck and enjoy that gun...

Prior to reading these posts, the barrel arrived and is as new condition. It fit up perfectly and it doesnt seem to bind in the forend at all. I called Browning and they were extremely helpfull. They said that I can use the full choke version of the Japanese barrel with 3" steel shot with no safety issues. He said that there may be some pattern issues though. As to the the friction ring set-up, Browning recommends setting it up to the 2 3/4" configuration when shooting 3" steel. That is where I am at now, so hopefully I have no feed issues.

Time is against you for this weekend, but if you can
get some different loads to try shooting into a piece
of card board to see if the slower loads will open up.
I've used cylinder choke with steel with birds in the decoys.
Steel holds together much better than lead.
I always set my A5's up for heavy loads first,
you have four settings on the friction rings.
If it don't eject, go to the next lighter setting
Easier on your shoulder that way. Got a picture?
Shotgun World' Browning section have more info about A5's
than you could imagine.

I did just what you did with my A5 except I bought the 26" barrel that uses the Browning choke tubes; it works great. The barrel is a little thicker than the original so I had to remove some wood (very little) from the forend.
C.G.B.

Took the gun out for an afternoon of duck hunting. With the lighter (2 3/4" high brass) load setting the gun functioned flawlessly. I am going to replace the second set of friction rings on my next outing as Brad stated in his post to see if it will cycle with the 3" steel loads. I am hoping that it does, because my shoulder is just battered. On my first shot, I forgot about my chest wader buckle and shouldered the gun on it. The recoil drove that buckle into my shoulder causing me excruciating pain on every shot after that. I did shoot my limit of 4 teal though. That gun is a killing machine....from both ends. I will get it adjusted though. I am now hooked on Brownings! A 20 gauge Magnum will be on my shopping list now.

I am bringing up this old post, because I felt like I wasn't doing this beautiful gun any justice shooting steel from the fixed full choke. I found out that Kebco (Ken Buch) sells Hastings barrels for the auto 5. I purchased a 30" which included a modified choke tube. Ken is a true gentleman to deal with and of course he is a member here on AR.
I will be selling one of the original barrels since no one needs three barrels!

I have a Browning A5 Magnum Belgium manufacture , with a 28in. modified fixed choke barrel . Ive shot many cases of 3in steel thru this gun , while duck and goose hunting . It patterns well with fast steel ,1550 fps # 2's