You can interchange barrels on a standard size 1100s, they will fit.
To put a 12 barrel on a 20 STANDARD receiver it will fit on there.
The ejection port is not big enough to allow 12 shells through it,
but if you are going to somehow tube the 12 barrel down to 20
you probably can make it work. You will run into some gas
porting problems. If the tube you install is short enough
to allow the gas to escape through the 12 barrel gas ports
it might even work. If it doesn't then enlarging the ports
would PROBABLY make it work. You probably need to cut
a slot groove in the sub gauge 2o sleeve so that the extractor
of the 20 gauge bolt can get its claw onto the shell. Yes
you could make this work, not sure why you want to, but
for some reason if you want to do it, it can probably be
done, and actually probably not too hard to do. Of course
you need a 12 gauge forearm after you are done, if you do
it. Actually you could just hog out some wood out of the
20 gauge forearm and it will work on a 12. The last wood
12 gauge forearm I used on my last 1100 (12 ga) was actually
a 20 gauge one, that I just removed some wood on and made
it work. One problem might be the thickness of the barrel
extension. A 12 PROBABLY has a thinner barrel extension,
and this could allow the 20 gauge shell to flop up too high
and this might cause erratic feeding of shells. This is just
a guess and it might well be the thickness is the same on
the two but if not then adding some material to the 12 extension
MIGHT be needed to get reliable feeding, but who knows.
If you measure the 20 and 12 extension it would give you a great
clue as to whether this might be a problem. Then also you
might have the problem of making the ejector tab of the
12 long enough to reach the smaller 20 gauge rim so as to
eject it. Actually you might be able to take a LW 1100
20 barrel and remove the gas cylinder, cut the barrel off
just in front of the gas ports, then turn down the barrel
to fit exactly in a 12 barrel like making tubes for an over/under.
drilling out the gas ports in the 20 way over size would
probably get them lined up with the gas ports in the 12
barrel that it will be sitting inside of. Pretty involved but
MAYBE possible. Probably also expensive, more costly
than just finding a good deal on a LW 20.
I have taken a 12 and put on a standard 20 barrel, and fired
it, also I used the 20 gauge bolt. It works, but of course the
20 gauge shells won't work in a 12 gauge magazine, effectively
you have a 20 gauge single shot gun that works. You are thinking
of going the other direction, somehow making a 12 barrel
sleeved to 20, also work on a standard 20 receiver. The
problem is similar, just a slight bit more difficult, but hey
if you like challenges then this one probably is not too bad.
All in all it is pretty difficult to figure out how this could be
justified, with all the expense etc.
