Look for a used 870 in 28 gauge.
There's an article in the new Shooting Sportsman. I think Bruce Buck wrote it, on subgauge adapters.
The words "Break action" crop up repeatedly.
Alla in all, the short adapters do work somewhat in break action shotguns, BUT patterns do not measure up to those fired in standard barrels or even the long tubes like those used in skeet.
You can also load down. Son's first shotgun was an NEF single weighing 5 lbs 9 oz and shooting a 3/4 oz load. Not a 28,a 12. I loaded down some 12 gauge loads to 3/4 oz and 1150 FPS. Creampuffs, but broke birds nicely inside 30 yards...