Jim,
Maybe Chris S. misunderstood the AR reference chart that you referenced. I just wanted to defend the comparison link on ARs you posted. It is accurate on the quality. Remember, this list is only for specific models from a particular manufacturer, but is also overall a good gauge of how that manufacturer makes things. If xyz manufacturer properly stakes the carrier key sometimes (as Chris pointed out, his was better than the Bushmaster listed), and not all the time, they aren’t going to get the “X.” This is because they do not consistently or reliably produce quality. I can tell you from my firearms teaching experience, that the little things do add up or are by themselves critical. Lack of quality can get you killed. In particular, on an AR type weapon the most important thing to make sure that it is done properly is the bolt carrier group. The gas key screws must be staked or they risk backing out during firing, which can result in a major breakage/malfunction or even risks danger to the user. I’ve seen this happen. Also make sure the extractor spring has the black plastic insert (to boost extractor tension) or you might experience extraction issues. I’ve seen a lot of ARs malfunction (type 3/double feed) because some company saved themselves 25 cents in manufacturing and skipped inserting that black insert on a $1,200 rifle! That kinda makes me wonder what other corners they cut. Honestly, things like parkerizing under the front sight base that are on that chart, aren’t critical, but they can potentially extend the service life of the rifle and are an overall indicator of the attention the company pay to the manufacturing process. Like Chris S., I own a Bushmaster. But I have upgraded and fixed some of those more critical things on it (I swapped to an LMT bolt carrier group, and I staked the castle nuts), so that it is TEOTWAWKI-worthy.
Chris is right that you have to be careful when buying an AR-type rifle from a private party because many sellers will tell you (either by honest mistake or dishonestly) that “this is an xyz brand AR.” I got suckered a couple times when I was more inexperienced and didn’t know what I was looking at. Know your stuff, or bring along someone who does. Despite the paper trail, often buying directly from reputable [manufacturing] companies like Noveske or LMT, is the way to go. I strongly suggest SurvivalBlog readers who own or are thinking of owning an AR platform weapon to read that link you posted again, and in particular, the explanations below the chart that discuss why those things can be critical. Oh, and don’t forget, the AR-series rifles and carbines run best when well-lubricated. – PPPP