I guess all your AR-15s are "obsolete" now. Time to start dumping your 5.56 rifles, lol
- Thread starter theLBC
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It's a good cartridge it just has limitations like everything else. There are a lot of newbies with no experience jumping on it and they will have a learning curve working up loads since it hasn't been out long enough for others to post usable data. It will take a while for manufacturers to get enough bolts and even extractors in stock to handle all the new users.
Hardness is by definition extremely closely related to strength; resistance to deformation. The trouble comes when people try to use a single term to convey something that is the result of multiple properties. In the quench/temper processes in steels minimizing temper keeps hardness and strength higher. What is lost is toughness (area under the curve in a stress/strain plot). In fact, with no temper at all after a quenched thru-hardening, steel in the martensite phase would produce the highest possible yield strength IF you were able to get a pure uniaxial tensile load on it. The reality of the situation, however, is that it's basically glass, and any uneven loading will cause it to crack because there is almost no yield before break to 'soak up' the uneven loading.
Toughness, on the other hand, is increased by the balance between the height and width of the stress/strain curve. Enough ductility to have some give while retaining as high a yield/tensile strength as possible. The Charpy test measures toughness. Tempering in steel allows a percentage of the martensite to reform into pearlite, which is much softer and weaker, but has ductility. The matrix of pearlite/martensite is what makes steel very tough, and there is a very wide band of physical properties that can be attained with the same material based purely on the heat treat. 4140, for example can have yield strengths in the 60-80ksi range all the way up to 200+ksi.
Another big contributor is fatigue life. The harder you make steel, the higher the ductile-to-brittle-transition temperature is. We live in a relatively narrow band of temperatures and most hardened steels qualify as "brittle" at those temps. Brittle metals fail by fracture and crack propagation. Fatigue failure can happen in parts that are cyclically loaded, even if the loading is UNDER the yield strength of the material. Luckily most steels have a fatigue limit-- a stress level much lower than the yield strength that if you never exceed, the part will never fail by fatigue. If I had to guess I'd say most AR bolts fail by fatigue (micro-crack propagation) more than simply over-loading and outright failing the material. Hence the reason why even if you don't see pressure signs on the cases you shouldn't exceed book loads in PPC variants, and why different bolt designs/materials have popped up.
I own a couple of ar’s in 6.5creed and a couple in 224valk and a couple in 6.5g I will make one in 6arc just because it’s so damn cheap to do, if I like the performance out to about 1200 then I may invest in something better than a ba barrel. I keep spare bolts on all my ar’s oddly enough the only bolt I have ever broken is a 223 bolt ...
It’s my guess hornady will use their marketing to make the round at least as successful as the Valkyrie (which shouldn’t be hard given how federal fucked that up).
It’s my guess hornady will use their marketing to make the round at least as successful as the Valkyrie (which shouldn’t be hard given how federal fucked that up).
Does cold hammer forging increase hardness?
Also, what steel will fail first due to heat(from high round count): SS vs chrome moly? And would this translate to whichever steel can handle higher heat, also mean that steel will make for a barrel that is more accurate at said higher temps?
I believe hornady manufactured this to replace the 5.56 but I can tell you there is NO talk about ditching the 5.56 soon. We did just adopt the .338 norma, .300 norma, and are keeping the. 308
Cold work of any kind (rolling, forging, drawing, bending) increases hardness. It does it a little bit differently than heat treat, but the end result is similar. Cold work reduces grain size in the material. The dumbed down version is that the smaller grains have more grain boundaries in a given volume and the grain boundaries are strong. In general, larger grains = softer weaker material. There is a limit, though, to how much cold work can be done before the part fails.
I'm not familiar with YS vs. temp charts of the commonly available barrel materials to tell you, but purely on gut instinct I would guess that 4140 or 4150 holds up better than 416 at high temp. Don't take that as gospel I could be wrong. At normal temperatures the CM variants have a higher strength potential because the "stainless" steels have a bunch of chromium in the mix. Iron and carbon are by and large what account for the strength/hardness. Replacing iron with relatively large percentages of chromium takes away some strength but the chromium forms protective oxides on the surface, similar to how aluminum does, and prevents corrosion. 416 is "martensitic", and can be quench/tempered just like traditional steels. Other stainless steels like 17-4 are precipitate hardening and that's a whole different ball of wax.
Accuracy at higher temp... I'm speculating, but I would imagine that uniform stress relief after cold work and/or heat treat, and barrel concentricity/uniformity will trump material selection. I can't give you an exact answer to that question, though. Maybe someone else here has more experience with high temp material properties. Young's modulus is going to drop with elevated temperature, as is yield strength, and your bore will be a larger diameter due to thermal expansion. None of that is conducive to accuracy.
I’m still in the camp of 5.56 for defense and 6.5G for hunting. I was going to build a 5.56 coyote rifle, but I’ll let the 6ARC teeth a little and likely pick up a proof carbon 18” 6ARC for coyotes with mid weight pills. The 6.5 (deer) and 6 (coyotes, etc.) will be great hunting cartridges, IMO. Thanks Hornady, I’m not a loader and this is right up my alley since I’m already into Grendel!
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