No o-ring i guess. size 13 fits. - SilencerCo Thread Protector with O-Ring Engagement

Thought these were like some of my other pistols thread protectors that have an o-ring which keeps them from unthreading during firing. Neither of these came with o-rings but a size 13 fits perfectly and makes it function as I intended. One of them had horrible threads cut and a large bur inside I had to remove and tune up. Kind of spendy for what you get, but otherwise great, wish they came with an o-ring more than anything.