I've got a set of 20 to 28 ga little skeeters. They patterned ok, no real problem there. From what I read about them , they do pretty good.
HOWEVER, the ones I have, since I went from 20 to 28, were on the thin side, and I used some HiVel loads, which is a no-no with these, (They say target loads only on the package). Well, they swelled slighty, and shells get stuck in them, have to punch them out, they don't pull out easily.
The 12ga to 28 might have enough meat to prevent this. At the worst, just use target loads. Can be fun. Easier recoil.
One thing to be wary of, when using smaller than gauge shells with a larger gauge gun: IF you slip a 28 in a 12 ga barrel without the insert, then put a 12 ga behind it you WILL have a BIG problem. I know it sounds hard to do, but ic can happen if you are the least bit careful.
Aother thought would be to have a 28 ga barrel fitted to the H&R...