There are three versions of the MPX.
Unless you know what version you have, you will have to check the threading on the barrel of your airgun.
The Gen 1 MPX works with our MPX adapter, and it has 3/8x16 thread.
https://donnyfl.com/products/sig-mpx-1-2-x-20-adapter-a52?_pos=1&_psq=mpx&_ss=e&_v=1.0
The Gen 1.5 MPX has an M11x.75 thread, and we can make a custom adapter using our MPX adapter.
The Gen 2 MPX has 1/2x28 thread and you will need to buy a firearms silencer and go through the tax stamp process.