3.1mm - I'm using these on blowback rigs with a size four Gamakatsu Specialist RX hook. I find that the size is just on the small size, but it works. With a bigger hook they would not fit.

I used the 3.1 mm rig rings and really liked them. I use large hooks and large snowman presentation so the 3.1 works well for those

I picked up the 3.1 rig rings and they are pretty small. They match my size 6 NGT hooks, but barely fit on a size 4 Gamakatsu. I'll order larger next time.

I purchased these in 4.4 mm to use in my first attempts at rigging a blow back rig. I should have gotten a smaller size, but there's certainly nothing wrong with the rings themselves.

I got these in 3.7 mm and cant see a seam or a join in the ring. I think they must be made without a seam. How cool is that !
I do wish they came in a cool little box like there swivels do as these things are Tiny but I put them in an old hook box and all is good.
Made a lot better than the price might make you think as are all Ambush products.