not sure this is the best spot for this question but I'm going to try.
let's say a person had two identical suppressors. except one is in 30 caliber and one is in 35 caliber. how would the noise and recoil reduction compare if both were used on say a 308? I guess what I'm asking is how much louder would the 35 caliber be and would it also be less efficient on recoil reduction?
Slightly - but the smaller and faster the lesser caliber, it will leak additional decibels out the front.
Running 6.8 .270) out of a .30 Omega, there’s not a lot (subjective) of difference in the sound level, comfortable to me without earplugs.
Running a .260 through the same .30 caliber can - there was a marked increase in decibels, and not a combo I’d not choose to shoot without hearing protection on.
IMLE - Having the correct caliber cap on the end of the can is most important if you want to reduce decibels across multiple rifles with one can.