I shoot my Diana.

I have had this gun checked out by a very competent gunsmith (Dave Norin), who has OK'd it for shooting.

When I measure the chambers of my Diana, they are 2 1/2". (Actually a bit longer at 2.58".)

I use http://www.rstshells.com/rst_classic_shotshells_shotshells.htm Falcon Ultralites in it.

Pete