I caused an under gassed KAC upper that ended up being a gas tube in the same condition described. The ‘bulb’ was worn off.
I was very surprised to see this. This upper had one variable. It had been through a couple days worth of carbine class in a very sandy range. It was a deep mid to fine beach sand brought in to cover the range.
We did a lot of drills that got a lot of sand in the action. I remember cleaning it out of up where the gas tube was and the BCG being covered. The sand had stuck to the lube and made an abrasive compound.
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