“I first saw Aguila 1 ¾-inch 12 gauge shells at SHOT a couple of years ago. They are, for want of a better word, cute. My first thought was, “these would make great cufflinks.” My second was to wonder how many would fit into the magazine of a pump gun.
“Apparently a lot of other people wondered the same thing — about mag capacity, not the cufflink thing — because the web is full of videos of people stuffing Aguila shotshells into their guns. They come in a 5/8-ounce load of 7 ½ shot at 1,150 fps, a buckshot load containing 11 pellets of 4 buck, and a 7/8-ounce slug at around 1,200 fps. You can fit a lot of them into a shotgun. I believe the record is 29, into one of those bullpup riot guns with two magazine tubes. However, the short cases don’t cycle well in some pumps as the short hulls double-feed or sometimes fall through some carriers and out of the gun.”
Shotgun Shells have a lot of empty space in them. Fancy shotgun shells have special wad cups that cushion the load as it’s being accelerated, preserving the shape of the shot for better, more predicatable patterning. But it’s still a blackpowder load; Minishells just nix the empty space.
Aguila Minishell shotshells are only 1.75-inches long, so they greatly increase the capacity of pump-action shotguns — provided your shotgun can cycle them. They work great in Winchester 1300 shotguns and by extension Savage Stevens 320s and the FN P12. The 1300 has been discontinued but there are reports that they work well with the 1300s successor, the SXP. Many have said that they can work with Mossberg 500s once you get the hang of how to work the forend. Remington 870s usually require modification.
They are also somewhat low-recoil ammunition and naturally work with all break-action shotguns.
Here’s some more video on these nifty, if a bit finicky shells.