The only listing of the little H & R 28-gauge hammer double I've found was in the 1912 SD&G catalogue --
The Remington Arms Co. Model 1893 single barrel was being offered in 10-, 12-, 16-, 20, 24-, and 28-gauge in their 1899 catalogue.
In their April 1897 catalogue the Model 1893 was only offered in 12-, 16- and 20-gauge.