If I was going to try this I'd get one of the alignment rods or whatever they're called. I bet someone on here is willing to measure one for you. I'll have my .223 end cap in a week or 2 but it'll be a hanson, and the name brand one might have a different orifice. Hmm, now you have me wanting both measurements, because I'm like you, I want it as quiet as possible. Wish the 223 one was smaller actually, as I've switched to all .224 projectile cartridges...Has anyone tried 6.5mm through the SiCo 556 End Cap? If it’s truly 0.323 then for a 0.264 bullet you’d have approximately 0.03 of clearance all the way around.
From doing some reading, it seems most suppressors have a bore that is 0.06-0.08 larger than the projectile recommended, so shooting a 6.5mm (0.264) bullet would theoretically be okay.
Not sure I want to be the first to try it, but also would much prefer to 223 end cap over the 30 cal end cap if I could use it with my 6.5cm.