I've been doing some research on the Hi-Capa o-ring since mine just went as well. It seems the o-ring that will do the job is a 10x2mm. (inner dim x thickness)
If I manage to find some I'll order 100 or so. I'll keep some for myself but make the rest available.

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"Two Wongs do not make a Right"
XT Team - Task Force Wong; Tai-i Wong
-TFW Springer Weapon Assault Team (S.W.A.T.)