If the shells were not so expensive then a 28ga would a be an excellent choice. Berreta and Browning both make it on a 20ga frame.
Reloading for the 20 and 12 is usually not all that cost efficient. Reloading for the 28 is.
Good idea, but you will probably get the same thing by going 20g and 3/4 oz shells. (3/4 seems to be pretty standard in 28g)
