I've shot 16's for about 30 years and stumbled across some things called "little skeeters" in the US recently. They are gauge reducing tubes that reduce the chamber. I bought some mail order to the UK, a set in 16 down to 20 bore, 16 down to 410 and 12 down to 410. They are recommended for use with only clay loads. A bit fiddly to get the empty shells out, but I can practice with 20 bore cartridges in the 16 and keep my eye in, the saving goes on number 5 bismuth for ducks.