Could any kind person confirm what foresight tunnel would fit onto an Air Arms MPR precision? I'm looking to replace the stock one with something like a Centra Score and am led to believe I need a M18 size, however they only seem to come in either FWB, Anschutz or Walther fittings, so I can assume there must be some difference between these, but can't fathom out what these differences are. The AA website doesn't mention anything regarding sizes.

I'm now questioning whether it's even worth changing from the stock tunnel and maybe the way to go is just changing the element. Any help or advice is much appreciated.