The 66 uses basically the same action as the 760, but the pump stroke is longer and it uses a longer sleeved barrel. It was rated with a little higher velocity than the 760. Originally all 66's had rifled barrels, but I think most of the last ones made (2008) were smoothbores. A lot of the parts of late model 760's are interchangeable with late model 66's. I had an old plastic framed 66 that I replaced the receiver with one from a late model (clip feed) 760.
I plink, therefore I am.