You didn't specify which model Mossberg. If it's a pump, anything goes. If it's a semi-auto, it's illegal.
Here's what the "assault weapons" ban says in a nutshell -
A semi-auto shotgun falls under the classification of an assault weapon if it has 2 or more of the following -
Pistol Grip
Magazine Extension
Bayonet Lug
I'm sure it says something about grenade launchers in there too.
Basically, you can have a mag extension OR a pistol grip, but not both. And we should all sleep better at night knowing that those dangerous pistol grips have been regulated.
Also, I believe that detachable mags on a semi shotgun are a no-no too.
I don't think the barrels will interchange between a M1 and a M3. I also don't think Benelli makes a 28" barrel for the M3. However, a Remington 870 barrel can be fit to a Benelli. I've done it. Give me a shout if you need help.
Thanks,
Lee
www.thearmsroom.com