You can't identify extractor springs by the number of coils. The drawings do not even specify the number of coils except as a reference which is variable. That means the spring maker tests the spring and adjusts the coils and OAL to achieve the lbs of force that is specified on the drawing. Also, each brand may use its own wire thickness and a different OAL.
I just measured every extractor spring Brownell's sells. I can say there is some scary stuff there.
The USGI spring from 1971 had a stress/strain ratio under 50% when compressed to 0.112. Those can last 18,000+ rounds. True, they only have 4 lbs of force at this height, but the o-ring will add in the extractor tension without having to resort to an extra power spring.
Why is an extra power spring bad? Because more force means more stress. Some of the springs from Brownell's had stress/strain ratios above 80%! If you monkey around with extra power springs, then change them every 1500 rounds.
The new M4 spring that is Copper colored is pretty interesting. It has exactly double the force of the earlier spring but does not have a huge amount of additional stress. If I were going to run a spring without an o-ring, I would use the new Colt M4 spring.
Or use an old-style 0.022 wire diameter USGI (people call this the '4 coil') with an o-ring and it should last the life of the bolt (but since it is cheap, might as well change it at 5000 rounds).
Now here is why I feel Chrome Silicon magazine, action, and extractor springs are BS:
A magazine with the standard 17-7 stainless spring may be stored fully loaded with 30 rounds indefinitely. Note that Chrome Silicon springs will not resist taking a set better than MIL-STD 17-7 magazine springs. A spring 'takes a set' when it reaches the proportional fatigue limit. The higher the tensile strength, the more it can resist taking a set. Here are the material properties of three premium materials*:
Chrome Silicon:
Minimum Tensile Strength: 235-300 psi
Max Operating Temp: 475F
Fatigue Properties: Fair
Music Wire ASTM A228
Minimum Tensile Strength: 230-399 psi
Max Operating Temp: 250F
Fatigue Properties: Excellent
17-7 PH Stainless
Minimum Tensile Strength: 235-335 psi
Max Operating Temp: 650F
Fatigue Properties: Good
As you can see, all three materials are about the same with the main difference being temperature, corrosion, resistance, and price. 17-7 costs 2.75 times as much as chrome-silicon ** so it only makes sense to give up the corrosion resistance if the price is much lower and you are not worried about rust.
The CS spring makers would have to believe they have something new and better and that the military is just behind the times or penny-pinching for not using it. That is not true, as CS is far cheaper than stainless springs. The reason the military has not changed is likely because they have actual engineers who work with facts.
*
http://optimumspring.com/technical_reso ... _wire.aspx
http://optimumspring.com/technical_reso ... _wire.aspx
http://optimumspring.com/technical_reso ... _wire.aspx
http://www.docstoc.com/docs/4884305/Typ ... CIFICATION
**
Fundamentals of Spring Design from the Spring Manufacture's Institute, page 13, table SM-8