After shooting my AR10 two weeks ago, I noticed that the rim on the casings was slightly deformed from the extractor. This was the first time I looked at the rims, so I don't know if this happened on previous outings. Is it normal for the extractor to bend the rim? Also, when I tore the bolt down, I either lost the O-ring for the extractor spring, or the rifle never had one. I've tore the bolt apart a few times, and honestly can't remember if the extractor didn't have an O-ring around the spring. I have spare O-rings. Should I use one? The barrel is 18 inches and has a mid length gas tube.