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So i found this bit of information that i thought was quite interesting regarding extractor springs on AAC's website, i know this is more focused towards the ar15/m16 family but it makes you wonder for the ACR. I was always told the the black insert and gold extra power spring with 'O' ring combo was the best and to try to upgrade from the blue insert and silver spring, but it looks like i could be mis-informed.

http://300aacblackout.com/resources/...al-Sep2010.pdf


EXTRACTOR SPRINGS
We have 5.56mm test data for the Bushmaster ‘4 coil’ extractor spring with a blue insert and an o-ring. Four uppers
completed a 12,000 round test with no extractor spring related failures. Two uppers completed an 18,000 round test
with no extractor spring related failures.
We do not recommend using extra-power extractor springs because they are under higher stress and will likely
break sooner. The included o-ring adds the desired extra extractor force without requiring a spring which would
have more force and hence more stress. Also, extra-power springs, when combined with an o-ring, may make the
extractor harder to snap over the case rim and may lead to function problems. If you do use such a spring, please
don’t use an o-ring and change the spring every 1500 rounds for mission-critical use.
Note that, again per our spring discussion, Chrome Silicon springs do not resist taking a set better than ASTM A228
certified music wire when used at temperatures below 250 degrees F.
 

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I remember when that TCTO came out for the "gold" extractor springs. We had bags of them and had to go through every M4 on post to swap them out. What a PITA. In regards to the M4 it was to overcome the additional pressure that's still on the case in the carbine gas system through initial extraction. Mid length gas compensates or this with longer dwell time and of course the rifle length doesn't have much in the way of troubles there.
 
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