Long story short: 18 USC 922(r).
On a shotgun a magazine larger then 5 rounds knocks it out of the "Sporting Purposes" class and you have to start worring about parts counts.
This page gives you an overview of the law (It was written for AK's but much still applies):
http://home.comcast.net/~navy87guy/home/922r.html
So if you wanted to make your Cobra in to a Hicap, you'd have to make sure it complied with the section on "assembling" a rifle or shotgun that couldn't be imported. I normally do this on rifles, but If I had to guess those Benelli guys find enough US made aftermarket parts to swap into their guns to lower the parts count under 10. That'll be harder on your TriStar just due to less aftermarket support.
Countable parts:
(1) Frames, receivers, receiver castings, forgings or stampings
(2) Barrels
(3) Barrel extensions
(4) Mounting blocks (trunions)
(5) Muzzle attachments
(6) Bolts
(7) Bolt carriers
(8) Operating rods
(9) Gas pistons
(10) Trigger housings
(11) Triggers
(12) Hammers
(13) Sears
(14) Disconnectors
(15) Buttstocks
(16) Pistol grips
(17) Forearms, handguards
(18) Magazine bodies
(19) Followers
(20) Floorplates
Of those your shotgun (Probably, I'm not that familiar with a Cobra) has:
(1) Frames, receivers, receiver castings, forgings or stampings
(2) Barrels
(6) Bolts
(10) Trigger housings
(11) Triggers
(12) Hammers
(13) Sears
(14) Disconnectors
(15) Buttstocks
(16) Pistol grips (not sure if a one piece pistol grip/buttstock counts as one or two parts)
(17) Forearms, handguards
(18) Magazine bodies
(19) Followers
That's 13 parts. So to Hicap it you'd have to find some enough domestically produced parts to get the number under 10. Mag bodiy, Follower, buttstock, pistol grip would be the easiest if you can find them. Maybe a forearm?
