Originally Posted by
lysander
Well, the o-ring isn't required for rifles and carbines with the copper colored springs . . .
Why the insert.
During chambering, the extractor snaps over the rim of the cartridge case, this is a relatively violent action and slaps the spring rather hard. In some cases, it can be violent enough that the spring goes into coil bind (goes solid). And as you know, a spring compressed solid on a regular basis will have a short, unhappy life.
The solution to this is that the spring rate be increased. The ways to increase the rate of a coil spring are: 1) larger wire diameter, 2) smaller coil diameter, or 3) reduce the number of coils.
Since the distance under the extractor is limited, a larger wire diameter (1) would just make matters worse as the solid height of the spring would be increased. The spring is already near the limit of minimum diameter (2), and even if you did reduce the OD some more, you run into stability issues, unless you redesigned the extractor/bolt interface. And, since the extractor spring has four (4) coils with closed ground ends, only two (2) of the coils are active, so there aren't any coils to eliminate.
So, we are in a quandary,we need a stiffer spring, but we can't make one that fits. The solution is two springs in parallel so the spring rates add, and the only way they would fit is nest them.
You could have a inner spring (like the AR-10/AR .308), But that would be a very small OD spring and might have stability issues, or coil interference. Or, you go the cheaper route and have a stiff rubber insert.
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