Air rifles with factory integrated suppressors are legal, but is it legal to build/install a suppressor for an air rifle that did not have one from the factory?
IANAL, but since a lot of air guns come with what they consider to be suppressors (and many are removable), and there is no paperwork or background check, making something similar for a specific air gun should be okay as long as it cannot be installed on a real firearm.
Or called LDCs Lead Dust Collector