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Posted: 7/18/2015 8:10:45 PM EDT
noticed these marks inside my upper while cleaning it today and was wondering if they are normal. They appear to be from the cam pin catching the edge of the cutout on both ends. The cam pin and BCG don't have any noticeable wear marks on them. Two of my ARs have these marks and two don't.
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IMO it's normal. It is just the cam pin wearing in and it will stop once it wears in. From the looks of it I'd say it is done wearing in already. I have a lot of uppers that look like that but no worse. This. It will stop wearing and won't hurt anything |
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Those marks are normal. You will often find them on the aluminum upper receiver, but won't find them on the bolt carrier or cam pin due to them being made of steel, as the steel is harder than the aluminum.
CY6 Greg Sullivan "Sully" SLR15 Rifles TheDefensiveEdge.com (763) 712-0123 |
| Maybe someone can elaborate, but I've read (and it makes sense) that those marks are from manually cycling the BCG. When fired, gas pressure pushed FORWARD on the bolt as the BCG moves BACKWARD... thus unlocking the bolt (rotating the cam pin) within that cutout. Where as, manually cycling lacks the FORWARD pressure on the bolt, so the cam pin hitting that cutout is what rotates the bolt (thus making that wear, and why piston AR's seem to suffer more from this, as there is no forward gas pressure on the bolt with them either even when firing). Is this correct? |
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Quoted:
Maybe someone can elaborate, but I've read (and it makes sense) that those marks are from manually cycling the BCG. When fired, gas pressure pushed FORWARD on the bolt as the BCG moves BACKWARD... thus unlocking the bolt (rotating the cam pin) within that cutout. Where as, manually cycling lacks the FORWARD pressure on the bolt, so the cam pin hitting that cutout is what rotates the bolt (thus making that wear, and why piston AR's seem to suffer more from this, as there is no forward gas pressure on the bolt with them either even when firing). Is this correct? Whether the bolt is being unlocked by hand or by the gas, the bolt cannot move backwards until the bolt lugs have fully disengaged the barrel extension lugs. So, the bolt always rotates through 22.5 degrees before starting to move back, piston or not. Those marks are made on the closing stroke. The resistance of stripping the round from the magazine tries to stop the bolt's forward motion, however, the carrier is pushing the bolt forward and this push comes through the cam pin slot on the cam pin. So the cam pin rides on the left side of the upper channel, and it tends to clip the back corner of the cutout. Cycling the bolt without rounds in the magazine (live or dummy) does not cause this as the bolt does not want to rotate until it stops when the bolt hits the back of the breech. |
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