not 18 round, the tube would be almost 4 feet long. The shortest barrel a “civvy” can buy is an 18” - 18-1/2”. With a full length tube that should hold six or seven 2-3/4” rounds plus one in the chamber.
A bit “unwieldy”, eh? :)
Yeah, did some looking, the biggest tube mag that I saw was 6-7.
The Russian Saiga-12 is an AK-style gas-operated 12-gauge shotgun with a detachable box magazine. (Unfortunately for California residents, Cali lists it as an "assault weapon"). You can get 5-round and 10-round box-type magazines, plus a 20-round drum mag