I have shot a 28 bore quite a bit.
I recently had to have some new barrels multichoked, for 2 reasons:
1. They were supplied with absurdly tight chokes.
2. All the research I found on 28 gauge chokes was contradictory - so I thought it best to get a selection and work it out for myself.
In consultation with Ivan Reid at Teague, I had 3 chokes made - 8 thou (3/8 on the Teague chart), 12 thou (tight half) and 16 thou (3/4).
The gun shoots fine with the 8 and 12 chokes on clays. I rarely use the 16 thou. If I take the gun game shooting I will probably shoot 12 x 2.
In earlier times I shot a Guerini 28 bore and from memory I did not put more than half (factory supplied) choke in that. It would kill any ‘sporting’ clay and 40 yard pheasants.