The definitions you seek are in 18 USC 921 (922 states prohibited acts, 921 has definitions). If you are also interested in the law about the shotguns which were reclassified by BATF as Destructive Devices, you'll need to check out the National Firearms Act, which starts somewhere around 26 USC 5845. There's also a lot of regulations promulgated by BATF to implement and carry out the statute.
18 USC 921(a)(31)(A)
The term ''large capacity ammunition feeding device'' - means a magazine, belt, drum, feed strip, or similar device manufactured after the date of enactment of the Violent Crime Control and Law Enforcement Act of 1994 that has a capacity of, or that can be readily restored or converted to accept, more than 10 rounds of ammunition;
18 USC 921(a)(30)(D)
The term ''semiautomatic assault weapon'' means - a semiautomatic shotgun that has at least 2 of -
(i)
a folding or telescoping stock;
(ii)
a pistol grip that protrudes conspicuously beneath the action of the weapon;
(iii)
a fixed magazine capacity in excess of 5 rounds; and
(iv)
an ability to accept a detachable magazine.
Also of note;
18 USC 921(a)(30)(A)(ix)
The term ''semiautomatic assault weapon'' means - revolving cylinder shotguns, such as (or similar to) the Street Sweeper and Striker 12;