Hard to beat the tritium XS Big Dot - its what I went with on a 12.5''.
It did require the bead that was on the Grizzly barrel be replaced with one of a different size (as it is installed over the bead - and the Grizzly bead was too large).
I can't remember the specifics - but I believe all the information regarding the proper sized bead was available on the Brownells product pages (which is where I ordered the XS BD and new bead from).
A little carefully applied JB Weld - and my XS has been bomb-proof through a couple hundred shells, including a number of 3'' slugs and buck.