Quoted: Brownells has new chrome-silicon extractor springs. I wonder how they fit into this picture. I wonder if they last much longer than the Wolff springs.
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I have some of those. IMHO don't bother. Wolff is made of rocket wire, has a thicker wire diameter, and like half a coil extra length.
The Brownells CS extractor springs are the same wire diameter as the current issue 4-coil extractor spring.
The D-fender is supposed to outlive the rifle, which is a good thing if you don't want the O-Ring coming apart in the middle of a range session/carbine class/competition.
The Crane O-Ring is NOT Buna-N/nitrile rubber. Adco has stated in the past that it is not Viton. If it is PTFE, then it is dyed black. Black fluorosilicone may also be a prime candidate. Remember that low temp up to high temp is required by the O-Ring (something like -60F up to 500F is most likely close to the required temp range).
ETA: It sounds like you are looking at McMaster's catalog of O-Rings. I've been there and done that, it appears the best choice out of their O-Ring material selection for this application is PTFE. It is rated Excellent in almost every category, will work from -100F to +500F. However, it is "white" so it won't look the same as the Crane O-Ring.
I am using the D-fender right now and it is a fairly hard material. When they say it increases extraction force 4x, they weren't kidding. Your finger nail won't even budge the extractor. With a Wolff XP extractor spring and black insert, I can lift the extractor a little using a finger nail or brass punch.
I used some aluminum snap caps to manually test feeding, chambering, and extraction with good results. The weather has been too lousy lately for any range work.