I've just got off the phone to Sportsman Guns after speaking to a very pleasant sounding Lady who advises me that I've
got to send my rifle in to them. They will not issue a gauge.
Apparently (according to her), S&W have only sent them
one single gauge (really?

) and said Lady has been told by her Manager that rifles have to be sent to them so they can perform the check. This is clearly at odds with your information that would suggest they have more than one.
Having read a large number of negative reviews for this Company, this would back up your theory of them taking advantage of the situation by charging S&W a fee for checking each one (I sincerely hope I'm wrong).
Having checked our rifles with a CMM & DTI, and obtained some very accurate measurements, I think I'll give it a miss.
Iain