...for lack of a better term.
I think I may have a lifetime supply now, this will keep me busy for awhile.
I have done some primer testing with the quad in the 7/8" MU plug, and a pretty good range session. I have also done some primer testing only with a very small diameter (thickness) 6.5mm O.D. x 4.5mm I.D. x 1mm thick with very good results. The latter is really about .040" thick and may work better in some of the tight headspaced 7/8" plugs. I will do some actual shooting with that o-ring the next time the Huntsman comes up in the rotation.
These are the o-rings I have on hand at the moment. From center (metric) then top and clockwise (1/4" O.D. x 1/8" I.D. x 1/16" thick) for the remainder, type of materiallisted at the end:
Black, round, 6.5mm O.D. x 4.5mm I.D. x 1mm thick, Buna-N
Black, square, Buna-N
Orange-Red, round, Silicone
Black, quad, Buna-N
Brown, round, Viton
Black, round, Buna-N
Green, round, Buna-N
Here is a couple shots of the Remington STS primers (Win W209 in center for control sample), note the front two were with the very thin metirc o-ring pictured center in above pic. They still maintained a perfect seal, but need to be further tested under load. Notice the carbon on the face because of larger diameter I.D.
I have done some measureing and fitting, the square cross-sectioned o-ring should be just about perfect for the 5/8" breech plugs. Especially for the guys with extra headspace. The Remington STS primers average right around .295" +/- .002" in my sample, if you need more length the Win W-209's are always .300" to .305" in my experience. Quad on left, square on right.