Coop,
I tried out the primers with the square O-ring last night. I was shooting some magnum loads accross the chronograph, so this wasn't the best test to test for seal, since most of the o-rings start to bleed with 120 gr loads of BH209. I did shoot three primers before heading out back to the range. All three stuck slightly and allowed some soot to bleed through between the primer and battery cup. These primers gave instant ignition, but velocities and accuracy was the worst of any loads I have ever shot with Blackhorn 209. In all fairness, the weather was hot, and once barrels heat up, I don't have the patience to wait long enough between shots. The primers bleed soot under load, and after the 12 shots I did take, the standing breech was covered in soot. Nothing like what you would get with one primer in a primer carrier, but very visable. All primers required the use of a decapper tool, so my theory was "all wet". I think these primers will only be used to pop a couple caps before loading from here on out.
The square o-ring was also a disappointment with these loads, I really thought this one may work best in the 5/8" plug. I am going out on another limb with another theory here. I think the square o-ring may be to rigid, it takes up more space than another o-ring, and therfore the primer gas is pushing past rather than expanding the ring and sealing outward as in the case of the round o-ring. It is FAR better than no o-ring however, and not bad at all. It is just not going to be "THE ONE" in my quest for no blowback with magnum loads.
I would imagine that with loads under 100 grains, that it would seal as well as the others, but that will have to wait for the next test. Sorry, but I don't think either of these will be on my short list.