So, what is the perfect washer ?
I know your asking upnorthbacon, but I'd like to interject for new members that an "O" ring itself isn't always the pat answer.
I have come across 2 foreams where the routing for the barrel underlug was so deep the "O" ring listed in the faqs did nothing.

There was No added contact, the Oring was simply in there filling empty space.
And aromakr mentioning a contact point up front is interesting also, (a T/C trick) that's covered in the faqs too, If I remember it was a Gent experimenting by adding a business card for pressure.
I you really wanna dial these things in, ya gotta goof around with it, but the cost of ammo nowadays can be a limiting factor for alot of guys.
I've had good luck with the RTV Bedding/Freefloat technique, but it's been the hand loading thats really done it for me.