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Originally Posted by NobodyJones
Okay, I am no expert, or even really very familiar with the AR-15, so pardon my ignorance with the following question. AR-15's have a milled aluminum lower, has there ever been a model built with as milled steel lower?
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Emphasis added.
Building on MarkCO's answer, I'd agree that the aluminium grade in a lower isn't consequential. That's just because a lower doesn't normally see a lot of stress.
While there are some lowers milled from billet aluminium, the "normal" ones aren't. Those are forged.
I am periodically tempted by some of the features I first saw offered in billet lowers, like threaded retention of the bolt catch and safety plunger. There is also an aesthetic difference. Billet lowers typically have a sharper, more stylish appearance. I shy away from them just because sharp edges probably aren't as easy to live with as a smooth forged item.
I've never tried a 6061 upper, and have only 7075 forged uppers. If I do find one I like well enough to buy, I'll find out whether the threads will take the full 80 pounds of torque; this has never been a problem on my forged uppers.
There is a video somewhere of a fellow simulating out of battery events with billet and forged uppers. He concluded that the splintering seen in the failure of a forged upper is more dangerous than the bending seen in the billet equivalent.