How do I tell if my Benelli M2 receiver is 19mm or 21mm?

[Marc1911]

Thanks in advance for any input.

 

I am looking to replace the recoil spring tube on my Salient Benelli M2 and the vendors offer both a 19mm and a 21mm receiver replacement tube. How do I determine which size receiver I have.

 

Also - any input on the Sure cycle system vs the OEM Benelli part?

Edited August 30, 2021 by Marc1911