So if any of u people have played video games like COD, Battlefield, army of two, etc... they have underbarrel shotguns are they legal and can they be attained for an AR? not wanting one just thought it would be cool to show off.

Any shotgun with a barrel less than 18" is a NFA item (SBS, or Short Barrel Shotgun), thus requiring the proper tax stamp.
Under-mount, or "masterkey" shotguns suck. They are simply a breaching tool, and are quite cumbersome when mounted. The 101st Abn fielding the M26 MASS (bolt shotgun), and it too is probably unwieldy.
A slung 870 works great, as do small sections of det.

A lot of guys also bought 37mm flare launchers to make their A1-A5 look like a 203 host. No NFA/stamp until you pony up and buy the true explosive ammunition, so just the "look" is only the cost of the launcher.

If someone wants to build a service repro rifle of a certain kit setup, I say go for it if it's cost effective. If you've also got tax stamps/long wait/big bro on top of purely cosmetic modifications, it's hard for me to justify it.

I'm not bashing you for wanting to have a look-a-like, just be mindful of the hoops through which you'd be jumping and the inherent cost of doing so.

ETA: pic of the m26

The "Masterkey" shotgun was developed mainly as a breaching weapon for SF teams; the weapon used is a Remington 870 shotgun with a 3+1 capacity; the receivers are mainly Magnum receivers, so they can use 3" loads and loaded with slugs to be used to open a feisty door (hence the knickname "Masterkey") by blowing off the locking mechanism and hinges without the opersator having to use a dedicated shotgun.

The buckshot round available for the M203 must really suck to be trying MasterKeys or the M26.

Gig 'em,

backbencher

How small???

...'befoe...