well i have a rattler in 30 cal , but ok yours is 9mm so your oring should be 9x1,5 or 9x2 ,
here is measurements of entire orings for my 30 rattler , the company should post it I had to measure every seal , so you can convert oring sizes with a convertor I used a American caliper to measure orings but if you remove shroud
View attachment 555616 and blow in barrel with bolt closed no air should leak if it is leaking at probe your oring is bar , more often the air leak is barrel clearance loosen 2 screws holding barrel and tighten slightly , do not tighten it or magizine wont cycle , if it is loose air will leak in your face , messege me if you have a problem LOU