To add to Pat Cascio’s comments on AR rifle construction, I thought I’d share the following:
This document explains the criteria.
This chart puts them all together
One of the very critical components is the buffer tube on carbines. The aftermarket tubes are of 6061 aluminum, versus 7075, and are milled, rather than being hammer-extruded. They are about half as strong as mil-spec, and have less gripping surface on the threads. This is probably one of the most critical areas of failure on the rifle.
Please note that Knight’s Armament is not mentioned on this chart, but they will happily detail the internal redesign they’ve made that from all tests and reports is superior to the standard design and materials. However, it is also much more expensive.
The AR bolt carrier group is easily replaceable, but it’s worth the extra money for the stronger components of tougher alloys to increase operating life. In addition, I differ from most and always recommend the hard chrome finish on the bolt carrier group. While on active duty in the 1980s, I got to handle both parkerized and chromed groups side by side, and there was no comparison. The Army went away from the chrome for several reasons, one of which was cost, but I believe this was a huge error on their part. The chrome finish is tougher, more durable, has greater natural lubricity. Heat treated and parkerized steel has a static coefficient of friction of about .8 (1.0 is the baseline). Hard chrome has a coefficient of .05. It actually performs better with minimal lube, as the surface tension of the liquid increases drag.
I will disagree with Pat on one point: It is certainly possible to get a very accurate and functionally reliable AR in the $600 range, but it cannot be as durable in the long term as one built with better materials, which will always raise the price. I would advocate an inexpensive rifle over none, but when opportunity presents, it should be assigned practice, range and backup duties, with better rifles taking the SHTF role. – SurvivalBlog Editor At Large Michael Z. Williamson