Fill out the ATF paperwork for the silencer for the 9 mm MAC-10 and take it along and talk about how much quieter it is than the .45 caliber Tommy Gun when you use the silencer!

That would get some panties in a bunch at the Market for sure!  
 
Here's  link to the ATF Form 4:

https://www.atf.gov/firearms/docs/form/form-4-application-tax-paid-transfer-and-registration-firearm-atf-form-53204/download