At one time 12, 16 and 20 gauge model 1100s used the same receiver. Ultimately the 20 gauge was changed to the smaller 28-410 gauge receiver. The 16 was always a full size or standard receiver except for the bolt and some internal parts.
The 16 and 12 ga receiver are both the same in the mossberg 500's too. Only difference is the marking for the 12 is 500A and the 16 is 500B.
There are some slight differences in some of the internal parts. Namely the bolt for the 16 has deeper milling in the bolt so the extractors will be tighter on the rim of the shell and work better. But it's not a huge difference in depth.
