This thread made me go to the shop and pull the O-Ring kit. Here goes FWIW (import stuff is bold and underlined)
***REMOVE O-RING to shoot suppressed!!!I repeat, DO NOT SHOOT YOUR SUPPRESSOR WITH O-RING INSTALLED ON BARREL*** (most cans are designed to index on the barrel shoulder for proper alignment, an O'ring prevents that metal to metal shoulder contact so you risk a baffle strike)
O-Ring size for over the barrel threads
Buna N (Nitrile) 90 Durometer (technical name of high pressure BOSS O-rings for heavy equipment) AA568A dash No. 905 / tube size 5/ Tube O.D. in inches .550-.556 using calipers. that's approximately 35/64 or 14.20mm. / I.D. in inches .414 (correction edit for the O.D., Thanks for the catch EASTMAN!!)
The next size smaller washer at 1/4" fits INSIDE the thread protector at the end of the cap between muzzle and barrel BUT ****warning, crushing this washer tight enough to secure the thread protector swells it to the point it WILL contact the bullet slightly as the round passes through the end of the thread protector. I would NOT advise going this route. (A thinner size tubing washer may work or taking an exact knife and trimming the excess O-ring MIGHT work)
A 3/16" size O-ring works great for dry firing (pinch it closed and place over the firing pin, but dont forget to REMOVE IT before you store the weapon!!)
