Ar-15 ftf & fte

Folks this is driving me nuts. First off I'm not a gunsmith, but I can usually figure things out. I have built several AR-15's, including 80% lowers, a Glock Armor and a decent shade tree machinist. I have a good working knowledge of the AR platform.

Several years ago I bought a Frontier Arms polymer lower. It came complete. The current upper will not extract the spent case. The bolt moves back 1/4". When it does extract, the bolt doesn't have enough energy to seat the next round. The barrel is a 16" with a mid length gas tube. I have replaced the gas tube, gas block and gas rings, extractor and spring. I just can't get the darn thing to run. I have scrubbed it clean and lubed it as per the "book". I have even tried a different bolt.

If I put a different upper on the lower, it runs like a Swiss watch. So it's not the buffer, tube or the spring.

It seems to me it is a gassing problem somewhere. I know they can be finicky. What am I missing?

If you are going to tell me to buy an AK, don't waste your breath.

Ideas?

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Ar-15 ftf & fte