Holosun 507k has a 32 MOA circle with a 2 MOA in the center.
Holosun 407k had a 6 MOA dot.
I personally love my 507k on mine.
Mossberg recommends Holosun on them and is what they have on display models at shows but there is other brands that have the same RMSc microdot footprint and will fit without an adapter plate such as the Shield or Swampfox Sentinel.