The product information page at Midway had this explanation for some difficulties..
Mini-shotshells are only 1-3/4" long, and greatly increase the capacity of pump-action shotguns. They work great in Winchester 1300 shotguns, though other pump guns may not cycle reliably without modification. Perfect for use as low-recoil ammunition for single-shot or double guns as well. This particular load features a combination of #4 and #1 shot.
Muzzle Velocity: 366 fps
Notes from Aguila on Shotgun Modifications to use Minishells:
First of all, any SAAMI/CIP approved 12ga shotgun may fire minishells, be it side-by-side, over-and-under or pump action; however, the latter type will not cycle them flawlessly. The out-of-the-box shotgun that we have tested cycling Minishells very well, is the Winchester M1300 18" Defender. Winchesters 1897 seems to like the shorter rounds, however not as much as the Winchester 1300. We still have to test the Winchester 1887.
MOSSBERGS 500/590s will cycle Minishells pretty well once the shooter acquires a "feeling" as to how to pump the action; however, this will not prevent the occasional jam. To further solve the problem, we have lengthened and widened the little short metal bar (shell elevator), which catches the round to be subsequently fed to the ramp and chamber. This modification will not prevent you from firing regular size shot shells. The other modification that would decisively help in the proper cycling, would be to install the ejector in a more forward position to catch the spent shorter shell and eject it, before it may flip and cause jams. However, we DO NOT recommend you to do this, because you will most likely no longer be able to fire regular size shells.
Any competent gunsmith should be able to perform the modification to the elevator described above.
REMINGTONS 870s seem to require a more involved modification, which we have never attempted. We have received calls from clients stating that they have tried our Minishells in their 870s with excellent results, and that they did not understand why we have always mentioned the potential cycling problems. To this, we would like to state once more that Minishells will not cycle flawlessly in regular pump action shotguns, no matter how well some customers are able to cycle them. Again, the Winchester 1300 Defender would seem to be the one cycling Minishells best.
We do not know of any semi-automatic shotguns functioning with Minishells.