Quote History Quoted:
As barrels have gotten shorter, cycling issues have cropped up. The Army technical data package (TDP) for manufacturing M16s and M4s was updated to include a new extractor spring insert (black), in place of the old one (blue).
The o-ring around the spring started as a commercial fix for failures to extract on short barrels, associated with opening up gas ports to cycle on weak, crap ammo causing extraction issues when you moved up to regular, hot milspec ammo loads. With a black insert and an o-ring, it's possible to run your over gassed carbine on a wide variety of ammo, but it does tend to be hard on the brass.
The military only ever runs M855, basically, so military M16s and M4s get by just on the black extractor spring insert.
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Information is spot on, but just need to add a touch more to it.
The blue or black inner inserts in the extractor spring was Colts markers for the tension off the extractor spring. The blue insert was a standard tension spring, while the Black insert is the marker for an extra tension spring for the M-4 rifles. With other manufacturers other than Colt offering extractor spring, is not longer safe to assume that the color of the spring insert is a true marker of the tension of the spring (other than if you get the extractor spring directly from Colt that still follows there own standard). Also, there is no difference in tension between a black or blue insert itself, and other than as a spring marker (if the spring is directly from Colt), the inserts sole purpose in the spring is to keep the spring from collapsing sideways. So having said this, it's no longer safe to say if your extractor spring has a black insert, it an extra tension extractor spring (other than if you have a rifle made of all Colt certified parts).
As for what started adding the #60 O ring around a standard tension extractor spring, it kind of like the chicken or the egg discussion, since the first commercial sold items to do this was the D-fender (O-rings where being used before the D-fender came out, but not being sold as a branded item for this use), and then other company followed such by commercial offering the #60 O ring as part of upgrade package with stronger springs after the D fender hit the market.
As for D fenders and O rings on standard tension springs, lets face it's, it a band aid to correct a problem elsewhere. If the rifle requires a extra tension unit (such as in the case that you are running heaver than 62gr ammo), then just a extra tension extractor spring with insert alone (without a O ring or D-fender) should be used. Using both a D-fender/O-ring and extra tension extractor spring together increases the tension of the extractor too much, and although such will keep the extractor from slipping off the case rim during the pull cycle, it cause problem on the feed instead. Hence the extractor tension is now so great that the extractor can not spring out to climb over the live round as loading to allow the bolt to lock up correctly.
To sum it up, you will see myself and other suggesting that a O ring be installed for rifle break in. The O ring will last for about 500 rounds, and once the O ring wear out, either the rifle has self polished in chamber through live fire and cleanings to no longer required the O ring, or you have problems in the rifle that should be resolved instead (and not just throw in another O ring Band-aid as a permanent item/part on the rifle).