Little bit of an aside but I would recommend micrometers over calipers. In my experience, even good calipers have more variability than you want in a precision part like a bolt.
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What's a few thousanths between friends? :p
Or a CMM if you want to get it dead right...just saying
^LOL At this point in time I just want to get my design into autocad, and then see if I can get someone to machine out a working model. I'm not planning on testing it yet, or even building one out of spec material or to exact specs. I have access to calipers, so I want to get some rough specs to start.
If anyone were so inclined to take some measurments on a quality, new/lightly used milspec bolt and carrier with the proper equipment I would greatly appreciate it though.
now THAT looks like the ultimate AR bolt, indeed! At first glance the third hole in the bolt carrier would seem unnecessary...but simply imagining the massive pressure the whole BCG is subjected to upon firing, another bleed hole cannot be a bad idea at all. One has to ask oneself why Eugene Stoner in his infinite wisdom did not think of such design tricks to begin with, but remembering two things about the entire AR platform could easily explain it all! 1 - Stoner was first and foremost NOT a small arms designer! His engineering background was in lightweight alloys and composites for AEROSPACE not guns after all, the difference between them could be thought of as that between chalk and cheese...and secondly, the AR 15 was a very hurried redesign of his superb AR 10, a much larger weapon with bigger receiver, barrel, chamber and BCG (notice all the photos of the earliest AR 10s, intended for competition in the US Army contest to replace the M1 Garand, had CHROME PLATED bolts and bolt carriers - indicating Stoner realized fouling was going to he a big problem). In other words, sub-par material strength for the carrier and especially the bolt, was guaranteed to be built in when the large BCG of the AR 10 was simply made smaller for the 5.56 round!
Being a complete AR noob that i am, i have never had a bolt fail. I suspect that many bolt failures occur in soldier weapons that get fired full auto quite a bit, the devastating combination of heat, dirt, and pressure surely plays hell on the bolt of any M16 or AR. That's a no brainer...
on aCould the AR USE a stronger redesigned bolt to fix it's flaws, certainly! Are most makers of AR rifles going to adapt the more expensive but stronger bolts, and are users going to retrofit? The short answer is almost certainly NO. THE current bolt works well enough that most people will see no need if it costs them anything..i have already solved this problem, answer, carry a spare bolt? It fits right in the hand guard after all...and i understand a lot of soldiers in afghan do just that...some even carry a spare BCG, and why not?
I think if you want to make the ultimate bolt, you are not dealing really with the metallurgy of steel. The 15,000 round bolt life quoted above seems as much ass it's possible to get from steel. And you are limited by the shape the bolt must be just to fit and function. For a really improved bolt, i think you are really going to be dealing with titanium, titanium alloy or some other exotic metals since the design limits of carbon steel in an AR bolt have probably been reached long ago.
The issue is price to performance. The LMT bolt is, well was before the craze 2-3x more expensive than a standard Bolt, But it did not offer 2-3x the life or improvement. The price increase has to be seen in performance upgrades, most wont spend 3 times as much for a product that is 1.5 times better.
Beefing up the body of the bolt (like the HK 416's) might strengthen the cam pin hole area (perhaps lengthening bolt life where the bolt often fractures and separates).
Not sure what can be done with the locking lugs. While both KAC and LMT cut away some of the lug area, not being an engineer I don't know how removing material enhances strength and longevity.
http://www.hkparts.net/shop/pc/catalog/HKN103672-2.jpg
I've never heard of a bolt carrier failing.