The Hebrew word for the act performed at a circumcision is "לימול". The root appears to be נ.מ.ל. Following the exceptional rules for roots whose first letter is נ, the causative participle and nominative should be "ממיל" (with the second מ דגושה). Why then is the universal word for one who performs the act of circumcision "מוהל" as if the root is י.ה.ל (e.g.), which can't possibly be the case?
I believe you have the wrong root: http://ericlevy.com/Revel/BDB/BDB/14/num108.html#t59 what נ.מ.ל gives. For the root see: http://ericlevy.com/Revel/BDB/BDB/13/mem32.html#t18