Mature Metal Circumference Measurement

After roughly nine months of living the chastity life with my CB6000, my Wife/Mistress and I feel it’s time to upgrade to a custom device. I’ve been following this forum for about that same period, and thanks to the advice here, we quickly settled on the MM Jailbird. Like everyone else, we hope to nail the measurements the first try, so we picked up a set of sizing rings. Here are my current choices: 1 3/4 inch oval base ring 2 1/2 inch length 3/8 inch gap The real sticking point for me is determining the circumference measurement; unsurprisingly, this is the only one that can’t be tweaked if I get it wrong. The guide states that when using the sizing rings on the shaft, they should feel snug but not tight. When I test the 1 1/4 inch ring, I have to coax it over my glans a bit, and once it’s there, it fits very tightly against my shaft with no gaps between them. The 1 3/8 inch ring slides on more easily, leaving just tiny little spaces at the bottom of my shaft. My CB6000 has a 1 3/8 circumference, though I need to use the stocking method for insertion. Skin from my shaft frequently pokes out through the device’s vent holes, but overall it sits very comfortably. All things considered, following the instructions points me toward the 1 1/4 ring, yet I can’t see going tighter than the 1 3/8 of my CB6000, which fits well. I’d love to hear from any of you who own MM devices; your help is appreciated.