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This is made of 6061-T6 aluminum.
Guntec's website says: "T6 aluminum", which likely means it's 6061T6 and not 7075T6, otherwise they would have said 7075T6, which is the actual Mil-Spec/Mil-Std material. Very few manufacturers follow the Mil-Spec when it comes to Receiver Extensions Tubes because of the tooling involved to hot extrude 7075T6. Its much easier for machine shops to fabricate the tubes using other methods than the call-outs for the Mil-Std process for the M4 Technical Data Package.
For the shooter who just wants to plink occasionally and keep it to 5.56 NATO with a smaller gas port and standard weight BCG, these tubes should be fine. A 7075T6 tube made to the TDP will be a bit stronger.