I'd MUCH rather have to finish the mag well area and stockthread

than try to get the holes located/straight for the firing group!95% AR 15 Lower Receiver, ATF Approved!!! Shipping Now! - AR15.COM

That link is from 2 years ago, and you can buy an anodized "80%" lower that seems to have the same machining done for 30$ less.

http://www.tacticalmachining.com/80-products/80-ar-15-anodized-lower-receiver.html

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The $30 isn't the issue. the issue is it's much harder to line up the holes for the disconncetor and hammer and mag catch than it ws to line up the buffer and the mags. Your's many times more likely to ruin it, after all your work and expense