There are military shooting teams, which could account for some of the odd ammo (like the 28 gauge). Others may have been purchased by FBI, DoD, etc for ballistics testing and longevity testing. Some of the round counts were fairly small.
I have a friend that uses one. It texts the pictures to his phone to save in the data charges rather than email. He has a att SIM card that has unlimited texts fat costs him about 10 per month. It's a pretty slick set up.
It might be cheaper to just get a new barrel from Mossberg. Send their customer service an email with what you are looking for. I bought a 22" mossy oak 500 barrel for $160 direct.
I'm curious nationally, how much ammo has been sold prior to manufacture. It seems everyone has turned off back orders because the demand is so high. I have 223 on back order from cabelas from December 20th.