Navy Arms Co makes the SRM1216. It is a semi auto bullpup shotgun that has a 4 tube magazine. At the Great American Outdoor Show, they had their latest version of the SRM shotgun. The SRM1228 is a 12 gauge just like the SRM1216 but it has been modified to shoot 12 gauge Aguila mini shells.
The number in the model name denotes caliber and capacity. The standard SRM1216 has a 4 tubes of four rounds of 2&3/4″ shotgun shells. When you fire the first four rounds, you have to manually rotate the 4 tube cylinder to load the next tube. Because it is manually operated, the SRM shotgun complies with a lot of ban states. The SRM1228 has four tubes and each tube holds seven rounds of mini shells. I have played with Aguila mini shells with my DP12 and found that there is a discrepancy between bird shot and slugs. The slugs have a slightly longer OAL which means you cannot fit as many. In my KSG I could get twelve rounds in each tube with bird shot. But only eleven rounds of slugs were able to fit.
Their latest version of SRM shotguns is the SRM1228. It has been tweaked to run the 12 ga Aguila mini shells. This involved tuning the roller delayed action and shortening the cycling of the bolt.
In the top photo you can the 1228 next to a 1216. The ejection port was made smaller for the mini shells and the bolt only cycles back enough to eject the mini shells.
The price is a bit on the high side as a normal SRM1216 can be purchased for around $1500. SRM is also making a California compliant version. The magazine tube is fixed but can be removed with tools. And since the tubes are rotated manually, it is considered compliant with regards to capacity.