I have never owned or used sub-gauge inserts, and feel they are mostly an unnecessary gimmick. If you look at the plastic shotgun wads available, you will see there is a bewildering number of styles. I just don't see how anyone could declare that all of them would effectively obturate and seal the bore without extensively testing all of them.
Some are little more than cups that contain the shot, while others have the more common flared section that we hope would flare out under pressure to contain the powder gasses and seal the bore. But among those wads with the flared section, there is also quite a bit of variation, and it can be seen that some would do a better job than others. And I don't think any at all would do well when going to extremes like shooting a .410 payload down a 12 gauge bore. I just have too much experience with pneumatics and hydraulics to think that a piston seal that is much too small for a bore is going to be perfectly and consistently effective at containing pressure without leakage or blow-by. Then, even if the front section containing the shot spreads out to fill the bore, there will be spots between the petals where the shot can lead the bore, and also become out of round due to friction against the bore. I'm sure the choke would pattern differently with this much shorter shot column too. Another consideration is the age of the wads. It is said that the plastic used in wads becomes much less flexible as it ages, and that this affects ballistics even in the proper size bore.
From what I've seen of those high speed images of shotgun muzzles at the moment of firing, I really don't think that would give us a lot of answers either. Here's a link to a site with a bunch of these images. It can be seen that some wads do show much more flaring out from the powder gasses than others. But you would also need to measure the effect upon velocity by firing several rounds of each shell and wad type using both a sub-gauge insert, and the correct size gun. Then you'd have to do extensive pattern board analysis with all available wads as well:
http://kuulapaa.com/home/highspeed/Misc.html![[Linked Image from i.postimg.cc]](https://i.postimg.cc/sgx6kTsC/Shotgun-shot-High-speed-image.jpg)
Anecdotal statements such as "hits seemed weak" or "they seemed to work well" don't tell us much at all. I just don't have any need for sub gauge inserts. If I want to shoot a 12 gauge, I grab a choice of several 12 gauge guns and the correct shells. Same for shooting 10, 16, 20 ga. and .410. It would be kind of dumb for me to shoot 20 ga. or .410 shells in a heavier 12 gauge gun when I have perfectly good smaller gauge guns sitting in the safe.
I suppose that substantially built sub-gauge inserts might permit safe shooting of smaller shells in guns that have pitted bores or chambers. They might come in handy for some almost unimaginable SHTF or survival situation where one had a 12 gauge gun, and only had smaller gauge ammo. Other than that, I suppose they could be a tool for those poor guys who are in such fear of their wives that they are afraid to just buy more than one shotgun. In that event, you might as well just castrate yourself.
