We have been using the Buna 70's and had good luck, but also just picked up some Fluorosilicone to try as well. The difference is twice the heat rating with roughly similiar elasticity (according to the o-ring engineer where we buy them)...

We also sell the D-Fender and the designers of it have posted good information here in the past about how they feel its better for this application and the testing involved in designing it...

Quoted: have about 5-6k on one of them and with a shot of brake cleaner it looks brand spankin new

Very good info...  

"Crane O-ring"...........................

I hear you, and agree. I'm on the 3rd naming convention for the item. When I had them listed as  “Extractor enhancer”  or "Extractor tune up", everyone kept asking if it was the "Crane O-Ring".

Been down this road sooooo many times.

Ok look, sometimes ARFcom guys are hard to understand.
With one post they say that it's worth it to save up the money to buy the best, no matter what it costs. With the next post they say to buy the cheapest thing available because it's just as good as the more expensive item.

I guess the argument might be, "Well, it IS what Crane produces, so that makes it the best, right?" Wrong.
And then you might say," That's pretty arrogant of you to think you know better than Crane".
Then I might say,"We invented this concept and the O-rings didn't work to the level of reliability that we felt was needed, and so we went beyond the O-ring to design the D-Fender. We then brought it to Crane and had it demonstrated and tested, We got a Combat Approval Rating. We have it patented. The next year, guess what happened? A 'Crane O-ring 'just happened to be 'invented' after they saw what could be done, and didn't want to pay our price, and couldn't copy it exactly because of patents."

How about that?

If you want the real scoop from the inventors of this whole idea, the O-ring will work. And it will work fine in many cases, and in many cases it will hold up for a pretty long time. In other cases, it won't. Do we know which cases will be good and which ones won't? No. The problem is not simply what material it is made out of. It is the basic shape.  The O-ring simply is not the correct shape to deal with the forces involved in this application. It will "squish" unevenly, due to the way the pressure is applied to it, and this causes it to be moved around to a place where it is pinched and broken. Then it has been shown in testing to migrate to an area which is very likely to bind up the gun. You should replace often to help ensure that it won't break at a bad time.

Will it work? Yes.
Is it cheaper? Yes.
Is it the best one available? No.

It's your dime.
But when you tell somebody to not buy a cheap TacPoint, and save up for an Aimpoint because it will serve better in hard duty, remember to consider what decision that you made in your extractor enhancement device, and why.  It's easy to tell someone else to save up an additional $300 for an Aimpoint, but did you save up an additional $11 for a D-Fender?

I know that this might sound like "tough  $12 medicine", but we're not somebody that "fell off the turnip truck last night". We invented this idea. We tested this idea in many forms including O-rings, and the D-Fender was better. We introduced this idea to Crane.  We know all about this idea. Think about it.