The silicone o-ring was purchased from Amazon. My Accura will not close on this o-ring without making the primer pocket deeper. My rifle will close on a metal washer that reduces the head space by 0.016". It will close on a o-ring that reduces head space by 0.020", and does seal and allow no blow back on an o-ring that has decreased the head space 0.015".

sabotloader suggested you might consider a stainless washer. This would work too. However, that involves turning the diameter of the washer down to some less than 1/4", and reducing it's thickness to no more than 0.016" or so, unless you make the primer pocket deeper.

If you have an Accura breech plug that is new, or in good condition, the blow back isn't much..........perhaps not enough to worry about..........not enough to go to any expense, or do a breech plug modification. Myself, i started down this road of breech plug modification, first, because i had a hang fire once in a great time using BH209, and second, because my breech plug suffered from flame cutting in the primer seat. When i think about it, it seems to me that it is possible i had problems with flame cutting on the primer seat, because i didn't keep the breech plug clean. I haven't been too diligent keeping my rifles clean. It is conceivable to me that a clean breech plug won't suffer as much flame cutting.

If you wish to modify your breech plug for the fun of it, i fully understand; i have truly enjoyed playin' around with my breech plugs, and all the new things i have learned, and experienced. Right now i would rather work on breech plugs than do any of the other chores waiting for me; it is fun, and interesting.