I'm asking based on the Avenger review Steve put up. He purposely installed a .30 moderator on a .22 cal Avenger.
Bore to caliber clearances make very little difference in sound moderation. I have not seen a science based study on projectile stability vs instability.
Over sized is much much better than undersized, btw
The problem comes in where you have a straw barrel, floating inside a shroud, and then have an adapter and the moderator screwed onto that. That's a LOT of places for tolerance stackup and a good bet that the moderator won't be concentric once you get it on. One reason why I absolutely hate shrouds. Often times a squishy oring is what's holding the barrel concentric within the shroud. And squishy doesn't prove to be consistent for me.
A properly threaded barrel with a well made moderator shouldn't have any issues with the caliber it's made for. Or a good fitting slip/glue on moderator.
But when you have three sets of threads of varying engagement, all bets are off IMO and you might want to err on the side of bigger.
I can't say how much difference in sound it makes, as I've always made great efforts to set my stuff up to not have adapters and all that, and to use the properly bored moderator for the caliber.
In a GOOD setup it shouldn't matter. But with all the funky shrouded stuff out today I think overbored mods will become more of a thing.
On the side of frugality, if you buy a 30 you can use it right down the line. As long as you have 1/2 x 20 threads , at worst you buy a thread adapter. This said by a shooter w/ 3 moderators. Thank the airgun dieties FX has finally come over to 1/2 ×20. Now if everyone would just come around to foster fittings on all their guns .
Now if everyone would just come around to foster fittings on all their guns .