The crush washer I was using was not deforming evenly, so it was not parallel or concentric with the bore. I replaced it with my last spare.
Suddenly I understand why suppressor manufacturers require the use of peel-washers.
It's odd how I went from having spares to being out of them. I used two on Becky's carbines. I've used two on Sabrina now. I used one on Kaylee when I ditched the slip-over. They are cheap parts, but shipping is almost always more than the parts so you wait until you have an order for something else.