Hi Ted,
My SXP is the defender model with 20" barrel that make us the minimum legal leghth of a shotgun in Greece (1meter). I bought it for HD but it become "my gun" in IPSC challenges here. I mostly compete in handguns but I'm having fun with shotgun competitions also.
The reason that I'm writing these is, that if you don't have any specific reason to have your bead exactly 3mm, you should reconsider and aim lower or higher with a stable bead (like what I'm proposing). It is stable and solid after 2-3 thousand shells.
Like in every shotgun without rear sights you should not expect the precision of a riffle, with a little exercise you can bullseye slugs in 50 meters with this fiber optic.
Finally if you don't find any solid bead don't turn back to the metal one. Put any optic fiber and buy a laser shell to put into chamber. There you can examine where is the laser and where is your aim in 5, 10, 25, 50 or 100 meters...
PS: please define, "neck of woods"
I'm not into that argo...