Two into one will go
I have a .243 and a .22/250 rifle. The .243 has a heavyweight sound moderator that fits the other. I have approval to buy a moderator for the smaller calibre and have my eye on a lighter weight “can” to the one I already use. Would I be breaking the law if I bought one in .243 for use on both rifles, or will I need to
apply for a variation?
It would depend on the precise wording on your certificate. Normally, if you have two rifles and two moderators on certificate it doesn’t matter which you use provided you are happy that the moderator functions properly on the gun to which it is attached.