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Posted: 9/13/2008 12:34:07 PM EDT
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OK so here's one that has me stumped. Hoping the collective has some thoughts that get me back on track. What about a barrel, could cause excessive case rim damage from the extractor. IN short the extractor claw is almost tearing a notch out of the rim. Why the barrel? Because this problem moved from one rifle to another, with only the barrel being common. Totally different upper, bcg, charging handle, etc. Thoughts? Also this only happens with fired rounds. You can chamber/cycle rounds all day long without excessive marking by the extractor. Fire those rounds and you end up with notched case rims. |
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It's a DPMS lightweight barrel - 16" carbine gas system. I've seen a number of issues with these barrels and out of spec chambers in the past. I suppose it's possible in this case, but with a well cleaned chamber and in spec rounds there appears to be no sign of tightness, nice solid thunk when letting a round fall into the chamber, easily drops free when turned upside down.. (yeah not real scientific, but usually accurate) As for gas system, I thought that might be the case, so I did pull the sight tower and replace with a low-pro block. Checking the port it might be slightly oversized, but neither the orignal sight tower or the gas black have made a difference in the symptom. The bright side is it has good positive extraction. The down side is wanting to reload your brass. |
Not really the case. The problem arises because the case expands and "grabs" the chamber walls when the round is fired. If the gas pulse is reaching the carrier too soon with too much pressure the case hasn't had time to contract back off the chamber walls. I many cases it's not a problem, but if the chamber is either a bit tight or rough, as Quib points out, it takes much more force to rip the case out of the chamber. Hence the damage.
Wouldn't make any difference, the amount of gas is regulated by the gas port in the barrel, not (usually) by the gas block. The easiest thing to try is carefully polish the chamber, I'm sure there's a thread around here somewhere. Also possibly an H2 buffer. The next thing I would is get an exact measurement of your gas port and see if it's in spec. If it's on the large size maybe an adjustable gas block would help. If you're mechanically inclined you can drill and tap the front of the gas block and make screw in vent ports to bleed off some of the excess. Or just send me the barrel to dispose of and get yourself a middy. 
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Let me clarify about a year and a half ago I ordered some of these barrels and ended up with two that had out of spec chambers, but they were clearly out of spec. Drop a round in the barrel and you'd have to bang the extension on the table to get the round out. It was just a mechanical error in the part and not one you could miss. However as it relates to THIS particular barrel, it has NO malfunctions... It runs like a beast. Except for notching the case rims. If Uncle Sam was buying my ammo I wouldn't bat an eye. But now that isn't the case and I'd like to reload my brass, this carbine generates unusable brass. But as for running right... it flies along. |
Exactly what I was getting to. |
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