BT-99 Adt'l Barrels

I have two '99s in 12 gauge. I wanted a good "guest gun" and tripped over one at a good price. Now the old soldier would like to give 28 gauge a try. Is it so simple that I could buy a 28 gauge barrel and use it on my trusty 99's? Are they avail..........do they have to be fitted? Sources, observations, discussion--please!

Don't think that a 12 gauge receiver will handle a 28 gauge shell...nevermind the additional 28 gauge barrel.