WOW...now that I'm looking at the pic closer, you might be on to something.
Can you tell us if there is a metal ring on the outside of the dome, kinda hard to see from those pics.
If the outer edge seals against the head itself, and Not the dome...that is your problem. You will smoke another Oring in no time. Small bore domes the Oring sits and seals on the dome itself.
Big bore, as noted, seals between the lip on the dome and the shell of the head...