You might have noticed the D-Fender extractor inserts or "CRANE" extractor o-ring upgrades available for AR-15 rifles in Brownells catalogs or wherever. You can use a pretty ordinary o-ring and get the same effect.
What you want, specifically is:
Viton AS568A Aerospace Standard ID Number: 006 Inside Diameter: 0.125 In. Thickness: 1/16 Outside Diameter: 0.250 In.That's that description from MSC Direct*, where I bought mine for $8.48 (in November 2009) for a bag of a 100. The part number from MSC is 09265067. McMaster Carr has them even cheaper, part number 9464k11, but I needed a 1 1/2" die stock, and MSC had those a lot cheaper.
The D-Fender and "CRANE" o-rings will probably cost you $3 or more each from a gun shop.
Installing the o-ring is pretty simple:
- Remove the bolt from the bolt carrier.
- If the bolt isn't clean, then clean it. :-)
- There are two pins in the bolt, one is solid, the other is a hollow roll pin. The solid one is the one that holds in the extractor. Push in on the extractor, and push out the solid pin. It should not resist too much, and no hammer should be required. The roll pin holds in the ejector and spring, leave that one alone, it is harder to remove and replace, and if you're not careful when removing the punch from the bolt, the ejector and spring may fly across the room, and you may never find them again.
- The extractor spring should be captured in a hole in the extractor. In the middle of the spring there should be a little rubber insert. If the spring is broken, get another one. If the insert is missing, or crumbled, get a new one. All you need to do to install the o-ring is to roll it over the extractor spring, and seat it up against the extractor.
- Reassemble the rifle and test fire. Make sure it still works. Just in case.
On a somewhat related note, I'm also told that a #41 O-Ring over the front takedown lug of the upper receiver will help keep your upper and lower receivers from rattling together if there is a sloppy fit.
* - MSC runs a lot of coupon deals, check retailmenot.com for coupon codes if you're going to order.