I don't know whether 20, 28 or .410 ammuntion fired through SideKicks (chamber inserts) form a seal or not, but I can tell you from first-hand experience that 20 and 28 ga. performs perfectly fine through them. I used Briley SideKicks in skeet competition when I first got started, and both my 20 and my 28 ga. averages were higher than my 12 ga. They worked fine. The .410, on the other hand, was, well, let's say "not fine".

Like the 28 Ga? Check it out:

28 Ga. Society