Hornady 6mm ARC "New" cartridge

[David Milisock]

You're confusing pressures and performance. A huge capacity case cartridge may be limited to under 40,000 psi whilst one with a fraction of its physical capacity is rated at 65,000 psi. The determining factor on setting pressure ceilings isn't case capacity rather case and/or firearm construction and strength.

We've long had cartridges with a duality of acceptable pressures. Initially, it was solely down to the cartridges being designed for weak 19th century actions. The classic example is 45-70 Govt with three pressure determined sets of load tables in manuals according to action type - Trapdoor; M1886 lever-action; Ruger No.1 / Siamese Mauser. The 19th century Mausers are similar with SAAMI having set modest pressure ceilings for US loadings to suit M1888/91 and M1893-96 military actions, but where today's handloaders (and European ammunition manufacturers too for that matter) routinely load them to c.60,000 psi pressures for modern rifles. 6.5X55, 7X57mm etc.

In more recent times, the phenomenon hasn't so much reappeared (because it's never gone away) but is becoming increasingly common principally due to people designing an ever wider range of new cartridges for the AR-15 platform and the increasing practice of using three case-head diameters with a standard size AR-15 bolt, with the case-head dia. affecting lug thickness and thrust back on the bolt face. The bigger the case-head the lower the pressure that the AR-15 can take hence the Grendel and ARC with their 0.440" dia. heads being SAAMI limited to MAPs of 50,000 and 52,000 psi respectively. As @gunsandgunsmithing says though, that's a firearm inspired limitation, nothing to do with brass or case strength and boltguns designed to handle 60,000 or higher pressure cartridges cope equally well with the Grendel size case-head, in fact arguably better compared to the next size up, the Mauser / 30-06 / 308 etc 0.473" dia. design.

What case size does affect is how much energy can be packed in behind the bullet at an acceptable MAP. This in turn directly affects performance as measured by MV and ME, but not in a linear fashion. On performance variations arising from case capacity / charge weight factors, there is a simple and usually very accurate rule of thumb. If all other factors (barrel length, peak pressure, bullet calibre and weight etc) are equal, then the % change in case capacity sees a % change in MV of a quarter of the capacity change. Using your capacity values, the 6mm Rem's 54gn capacity is 21gn or 63.64% greater than the ARC's 33gn, so at the same pressures will see an ~ 16% increase in ME all other things being equal. So if the ARC gives a particular 6mm bullet 2,800 fps in a say 24-inch barrel, you'd expect approx 16% higher MVs from the 64% larger 6mm Rem, in this case ~3,250 fps also from a 24-inch barrel. That won't of course apply to the SAAMI ARC loading or handloads for AR-15s as they're firearm limited to a lower pressure.

In terms of ft/lb ME per grain weight of powder that makes the ARC a much more efficient machine than 243 Win, 6mm Rem or Creedmoor and similar as @gunsandgunsmithing says when loading for boltguns. The '25% rule' only applies too to designs that are reasonably efficient, that is not too small a case to be unable to hold enough powder to achieve peak pressure and at the other end not so over bore capacity that huge charge increments only produce small MV gains.

The growing popularity of the AR-15 platform for civilian range use and hunting aside, what has changed the playing field on this issue is modern propellant technology and the huge range of powder types, burning rates and specific density/energy levels now on offer. A generation or two back the ARC and Grendel would likely have been relatively poor performers simply because the available powders would have seen the case over-full long before peak pressures were achieved. I've no experience of the ARC or Grendel based 6mm wildcats, but a fair to middling bit with its bigger 6.5mm Grendel brother and powder selection is key. There are many suitable grades available though - Viht N133, Ramshot Tac, Accurate-2520, IMR-8208 XBR, Hodgdon CFE-223 and others - that pack enough energy into the little case to achieve good results.

I understand your concept however performance with lower pressures which is what we see in the 6MM ARC requires a powder not only with a specific burn rate but a case of sufficient powder capacity to maintain burning powder so as to continue the pressure for the entire length of the barrel. Increase that powder burn rate to increase pressure you may run out of powder before you run out of barrel. Although you usually can get more of a faster powder in a case then you can a slower powder.

Change that perfect scenario and the case with the larger capacity gains the velocity edge assuming the same length barrel. However efficiency goes down.

Case in point for 25 years I've loaded for the 358 Norma Magnum a 64,000 PSI short magnum case with a capacity of 88 grains of water and with projectiles 250 grains and under I can match the 358 Shooting Times Alaskan. In fact I can exceed the 250 grain 358 STA velocities with the Norma by 200 FPS. A more efficient combination. The 358 Shooting Times Alaskan is a 65,000 PSI case with a capacity of 105 grains of water.

When we up the bullet weight we now have to slow down the powder ergo we need to have more case capacity to maintain or increase velocities. As we go to and exceed 300 grains the 358 Norma falls behind the STA by 200 FPS. A less efficient scenario.

The 6MM ARC is at 2,750 FPS in a 24" barrel at 52,000 PSI and my 6MM Remington is at 3,270 with a 26" barrel at 65,000 PSI same bullet weights That's a 520 FPS increase with the 6MM Remington because it has an additional 13,000 PSI, 2 more inches of barrel and enough powder to continue burning at the needed pressure for the entire barrel.

In a bolt rifle the 6MM ARC may be able to be loaded to higher pressures but can it be loaded 13,000 PSI more? If it can be loaded to 65,000 PSI will it have enough powder to sustain the pressure curve? Maybe we'll see that it can produce 6MM Remington velocities with lower pressures and less powder. My bet is no, I think the odds are against it.

None of this takes away from the fact that the 6MM ARC is an efficient case very well suited for the AR 15 platform. So much so I'm buying an AR 6MM ARC. How the efficiency plays out in a bolt rifle remains to be seen. I'm keeping my 6MM Remington.

 

[Laurie]

In a bolt rifle the 6MM ARC may be able to be loaded to higher pressures but can it be loaded 13,000 PSI more? If it can be loaded to 65,000 PSI will it have enough powder to sustain the pressure curve? Maybe we'll see that it can produce 6MM Remington velocities with lower pressures and less powder. My bet is no, I think the odds are against it.

There's certainly no question of the ARC matching the 6mm Rem in terms of achievable velocities. That's why one burns over half as much powder again as in the smaller cartridge and accepts a barrel life that'll likely end up somewhere around half that of the ARC's. That's not something I suggested. In terms of outputs v inputs the larger cartridge will always be less efficient though - that's the cost of achieving the desired external ballistics. For that matter, there's hardly anything around that improves on the .22LR's ME per grain weight of powder ratio - but the 22LR's ballistics are rather constraining.

Whether one could load the ARC up to 65,000 psi in a good bolt rifle is an interesting question though. The European CIP Maximum for its sister 6.5mm Grendel is 4,050 bar / 58,740 psi, a fair hike over the US SAAMI AR-15 inspired limit. I'd imagine the ARC would accept similar pressures happily and small bolt actions would have no trouble handling that level, but 65,000 with a smaller case-head than those using the 0.473" dia. design, well ...... ???

I know that BR competitors load the parent PPC with Lapua brass pretty hot, but I've never seen a pressure measurement. I'd bet somebody (or six) on this forum has a fair idea though.

 

[David Milisock]

Here's a link to a 6MM ARC video.

I replaced my first barrel on my 6MM Remington at 3,000 rounds, I expect the 6MM ARC can go 5,000. Now I whack crows at 500 yards, BTW I only count one shot one kill shots, shooting 6 times to make a hit is not my thing. I usuall take both my Howa 1500 in 223 Remington with a custom load, on exceptionally calm days good for crows to 300 but have the Remington 700 BDL in 6MM Remington for windy days.

The HOWA in 223 shoots a 40 grain VMAX moly coated at 3,900 FPS, I use neck sized Malaysian military brass to get more powder, the bolt opens with my pinky finger. The 6MM Remington uses a 75 grain VMAX moly coated at 3,900 FPS, neck sized Remington brass again the bolt opens with a pinky finger.

6MM ARC bolt rifle pressures don't worry me as the new bolt guns are strong. I am certainly interested in the self loading factory performance. I'm a patient shooter, I won't take bad shots and the AR (as we say in the States) is the cats meow!

 

[Hohn]

I understand your concept however performance with lower pressures which is what we see in the 6MM ARC requires a powder not only with a specific burn rate but a case of sufficient powder capacity to maintain burning powder so as to continue the pressure for the entire length of the barrel. Increase that powder burn rate to increase pressure you may run out of powder before you run out of barrel. Although you usually can get more of a faster powder in a case then you can a slower powder.

Apologies if I'm misconstruing your comment.

I think you're suggesting that powder is burning all the way down the barrel. My understanding is that powder is almost entirely consumed very close to the breech. Once the peak pressure is obtained-- all of the potential energy from a powder charge has been delivered. From there on out, it's just a depressurization event, as that fixed volume of high pressure gas is allowed to expand and cool. The larger the volume of gas stored at peak pressure (i.e., the later it occurs down the barrel), the more potential energy is stored. But also, the lower the expansion ratio because there's less trapped volume ahead of the bullet relative to that volume behind the bullet. This reduces efficiency. Consider that a 9mm handgun with a 4" barrel can deliver 325 lb-ft of muzzle energy from just 4gr of powder. A .223 will have ~3.5x the muzzle energy. But it will burn about 6x the amount of powder AND require a much, much longer barrel. What would those ratios be of comparing the .223 from a 4" barrel? Pretty abysmal for the .223.

If the pressure peaks earlier, then the expansion ratio is higher. Higher expansion ratio means more efficiency. But it also means a faster decrease in pressure and a lower AVERAGE pressure acting on the bullet in a long barrel. Bore size plays here also as it's a major element of the ratio of volumes before and after the bullet. This is why some small, low pressure calibers gain almost nothing from a longer barrel. (think 45ACP in a long barrel).

What I'm suggesting is that if you have 52ksi of pressure peaking when the bullet is 4" down the barrel (for example), then the amount of powder it took to get there is immaterial. We burn powder to produce gas, hot gas does the work. I'm also suggesting that you will not have powder continuously burning and contributing meaningfully to the work delivered to the bullet. This is why the pressure trace from Quickload has the shape it does--early peak, steady and logarithmic decline from there.

This decline is inevitable, you cannot "maintain the pressure for the entire length of the barrel" because powder is consumed early- there is no additional energy being delivered to the system after the early peak pressure.

You comment about a case "requiring a specific burn rate" is true of all cartridges. Every cartridge is either capacity limited or the burn rate is too fast to allow complete fill for a given MAP.

 

[David Milisock]

Powder burning is an interesting discussion. If we assume the expansion and decompression the two cases loaded to the same pressure but different capacities would produce really similar velocities but they don't.

A few interesting observations, the 30 Carbine in a 30 Carbine and 5.56 nato in an AR. Can be loaded to peak pressures and in fact slightly over pressure but have insufficient pressure at the gas port to cycle the action with small volumes of fast burning powders.

Change the burn rate to a slower powder, increasing powder volume and you can lower the peak pressure significantly and the actions will have enough pressure at the port to cycle. It's the length of the pressure curve demonstrating that pressure builds in relation to expansion ratio, powder volume and burn rate.

I'm not sure what site I saw this but a girl did a high school experiment with velocities verses barrel length in a 308 Winchester. The discussion was the average was 27 FPS loss per inch and in fact she started at 26" shot and clocked, and reduced the barrel1 inch and shot and clocked again, continuing until she was at 16". A reduction of 10" and indeed average 27 FPS per inch loss.

However a closer examination reviled a look at the pressure curve. The loss from 26" to 25" as well as 25" to 24" was 50 FPS per inch. A loss of 100 FPS, the next loss of velocity per inch over the next 6 inches varied fro 9 FPS to 35 FPS, then at 18" to 17" and 17" to 16" returned to a loss of 50 FPS per inch.

Clearly there was a drop in pressure from the initial ignition from 16 to 25 inches and a resurgence of pressure the last 2 inches of barrel.

In her test a barrel more than 18 inches was near useless until it was 24" long.
Just one test but a clear physical demonstration of the pressure curve. You don't need more powder to reach peak pressure, you need more powder of the right burn rate to maintain the pressure.

 

[Hohn]

I'd like to see such a test repeated as it is not consistent with the many other cut-a-barrel-down tests floating around the web. https://rifleshooter.com/2016/02/6-...el-length-on-velocity-cutting-up-a-creedmoor/

I'd be wary of citing a test by a HS kid as evidence of powder still burning when the bullet is far down the barrel.

Powder burning is an interesting discussion. If we assume the expansion and decompression the two cases loaded to the same pressure but different capacities would produce really similar velocities but they don't.

A few interesting observations, the 30 Carbine in a 30 Carbine and 5.56 nato in an AR. Can be loaded to peak pressures and in fact slightly over pressure but have insufficient pressure at the gas port to cycle the action with small volumes of fast burning powders.

Expansion and compression of two cases loaded to the same pressure but different capacities will of course produce different velocities because the location of the bullet when the peak pressure occurs is different. The larger capacity case will have more stored energy because the peak pressure occurs with more volume behind the bullet. It will also have a lower expansion ratio, producing a higher average pressure for the identical peak pressure. Peak pressure is what he brass sees; avg pressure is what determines MV.

Port pressure in a semi auto is only one of a few factors in determining how the gun will cycle, and it cannot be evaluated without also considering plug time and gas flow (orifice size).

Absolutely a really fast powder in 5.56 or 30 carbine can be loaded such that the action won't cycle fully. But you won't see nearly as big a difference in muzzle velocity as you might think if judging pressure just on action cycling.

 

[David Milisock]

The web site you posted has updated data that I read, the overall change in velocity on the 9" removed from the barrel was remarkably similar to the girls science test between 20+ FPS to about 27 FPS per inch. His processes were no better or worse then hers. She was supervised by her grandfather who was a smith. I hope she got a good grade.

What is obvious is the very possibility that the 6.5 creedmore pressure curve was more symmetrical than the one for the 308 winchester, possibly due to match grade ammomused for the 6.5. That was indicated by the readings over 6" of the barrel between 16" and 22" having significantly reduced velocity changes of the 308 test. Surely neither of these tests are perfect.

What may have happened with the 308 was the projectile acceleration and increased expansion area created by the bullet movement by the 16" mark on the barrel exceeded the ability of the powder charge to maintain the pressure. Then the slowing projectile acceleration over the next 6" allowed the burn rate of the powder charge to recover pressure accounting for the more than doubled per inch acceleration for 24 to 26" barrel end.

Take a 30 06 with a short barrel 18 to 20" load up a max charge of 3031 and a 130 grain projectile. Then do the same with 130 grain projectile and 4831. Now shoot both loads at deep dusk, believe me you'll see how much powder is still burning at the end of the muzzle.

As far as gas port pressure the gas system is the same for all powder charges when testing the same rifle, ergo mute.

So the pressure curve either produces enough pressure at the port to cycle the action or not. It's why gas systems are so sensitive to burn rate, the 30 carbine rifle only functions with powders of burn rates from Enforcer to 4227 even though there are loads using Herco. The question is why work with Li'l Gun but not Herco? The slower powders work because as the bullet goes down the barrel the powder continues to burn over time, continuing to expand gases to maintain pressure until the bullet passes the gas port. Herco burns up before the bullet passes the port and has insufficient pressure at the port to function the action even though the projectile has reasonable velocity.

Smokeless powder is a propellant, not an explosive and burns of a period of time, albeit under pressure a short time, about 1 millisecond.

Powder burn rates and flame temperatures are subject to the pressures built up after ignition. For example Li'l Gun works well for 40,000 LBS pressures in rifles and handguns in fact I like it for my 30 Carbine because the barrel runs cooler under rapid fire. However at 55,000 to 60,000 LBS the flame temperatures have been reported so high it melts revolver parts. Similar can be said for all powders.

Modern powder manufactures control the ignition and burn rates to produce a more flattened (sloped ignition and longer pressure curve) as well as improved flame temperatures. You've probably smelled some of these chemicals used to control the burn speeds akardite smells like urine.

Here is a link to some pressure curve graphs more modern powders have a much more sloped ignition, a more stable peak and longevity to the curve as they have controlled the burn better.

https://www.google.com/search?sxsrf...iw=1261&bih=1741&dpr=1.5#imgrc=PP8GFiMJMx0r_M

 

[243Mendoza]

Apologies if I'm misconstruing your comment.

I think you're suggesting that powder is burning all the way down the barrel. My understanding is that powder is almost entirely consumed very close to the breech. ...

Not according to QuickLoad:

 

[Dusty Stevens]

Not according to QuickLoad:

View attachment 1186194

And not according to the differences in the sound when you get it right and go past that or the fireball/ no fireball as load changes too. Being at a short range br match you can tell by the sound whos burning what powder and how stiff the load is

 

[HomeSlice]

It seems to be the fad today to take something small (ARable) and try to make it do something big. 90% of the people who buy this don't own a chrono, so as long as they can hit a deer in the ear or Coke cans at 1K yards, it's all good.

honestly, if I can reliably hit coke cans at 1k with an AR I’d be happy to toss my labradar...

 

[SPQR70AD]

Not another 6PPC nock off!! We already have to many of them, and they are NEVER as good as the PPC. Just make a frigin PPC, how hard is that? Good news is though is that there will be more rifles that can be converted to PPC.

By the way I find it ironic that everyone is always trying to fit an AK based cartridge into their AR but no one is ever trying to fit a 556 into their AK.

in the early 80's I had a norinco AK in 5.56 cause there was no ammo for the 39. soon as 7.62x39 AK's came available I sold it and got a 39

 

[David Milisock]

It seems to be the fad today to take something small (ARable) and try to make it do something big. 90% of the people who buy this don't own a chrono, so as long as they can hit a deer in the ear or Coke cans at 1K yards, it's all good.

I agree the hand loader both tech savy and non tech savy will have a field day. I will hand load as well as use factory ammo. I'm not looking to lift her skirt up for a peek, I'm looking for that 52,000 PSI performance level for rapid fire work long and short range. Most likely will have a rifle with 2 uppers, a shorty maybe 16" with handle and combat iron sights and a 24" with a flat top and optics , maybe 2 rifles.

 

[David Milisock]

Not according to QuickLoad:

View attachment 1186194

My understanding is that the shape of the cartridge, ergo the powder column, short height but specifically the diameter in relation to the bore diameter helps control the leading edge of the pressure curve. Also enhance and extend the pressure curve. I'm really anxious about that 52,000 PSI performance, if it's repeatable as documented by Hornady it's thevdata meow.

 

[David Milisock]

And not according to the differences in the sound when you get it right and go past that or the fireball/ no fireball as load changes too. Being at a short range br match you can tell by the sound whos burning what powder and how stiff the load is

Over bore loads are sure noisy and dirty.

I'm a varmint guy and sacrifice a bit for increased velocity. Shooting 500 yards or more over terraced farm land in wind benefits from a reduced time of flight.

I'm a bit surprised to hear what you say about the bench rest matches. I would have thought that action truing, case and load preparation would have been everything. I would have not expected out of balanced loads at a match.

 

[Hohn]

And not according to the differences in the sound when you get it right and go past that or the fireball/ no fireball as load changes too. Being at a short range br match you can tell by the sound whos burning what powder and how stiff the load is

I don't think fireball is an indicator of powder burn per se.

I'm thinking a flash hiders wouldn't work if the flash was unburned powder because they cannot keep a hot fuel source from encountering combustible oxygen outside the barrel once ejected. TO the contrary, flash hiders work by exposing the hot gas to cooler ambient gas. If that hot gas was mostly unburned fuel, then this mixture would cause secondary burn (and flame) and make flash worse because the hot gas is well above the autoignition temperature of a powder.

Flash is overwhelmingly the result of hot gases that have already combusted. Yes, unburned powder to the extent it exists) is also part of it. But it's a small part.

Long barrels have less flash because they gases are cooler from expansion.

Many hot gases have emissivity just like steel does. Hot steel glows yellow orange even though it's not being oxidized. Very many materials will glow and emit light when heated. Including many gases and vapors (which is how all sodium and mercury vapor lamps work, as well as all "neon" lights and HID lights).

Sound difference are completely a result of pressure phenomena, not at all dispositive with respect to powder burn.

I don't doubt you can observe differences in powders and charge weights-- but your observations are explained for the most part by factors that are far more significant than the trace amount of unburned powder that might remain.

 

[Dusty Stevens]

Over bore loads are sure noisy and dirty.

I'm a varmint guy and sacrifice a bit for increased velocity. Shooting 500 yards or more over terraced farm land in wind benefits from a reduced time of flight.

I'm a bit surprised to hear what you say about the bench rest matches. I would have thought that action truing, case and load preparation would have been everything. I would have not expected out of balanced loads at a match.

The out of balance loads are the ones that sound like every other gun. A good tune has a sharp crack

 

[gunsandgunsmithing]

My understanding is that the shape of the cartridge, ergo the powder column, short height but specifically the diameter in relation to the bore diameter helps control the leading edge of the pressure curve. Also enhance and extend the pressure curve. I'm really anxious about that 52,000 PSI performance, if it's repeatable as documented by Hornady it's thevdata meow.

Theoretical, much more so than factual. Anecdotal at best.

 

[gunsandgunsmithing]

Over bore loads are sure noisy and dirty.

I'm a varmint guy and sacrifice a bit for increased velocity. Shooting 500 yards or more over terraced farm land in wind benefits from a reduced time of flight.

I'm a bit surprised to hear what you say about the bench rest matches. I would have thought that action truing, case and load preparation would have been everything. I would have not expected out of balanced loads at a match.

It takes an experienced ear to even know what Dusty is talking about. Comparatively, it happens sooner than later but you dont get that in a field but you can still learn to hear what it sounds like, with time.

 

[David Milisock]

The out of balance loads are the ones that sound like every other gun. A good tune has a sharp crack

That makes sense as 99.9% of rifles are shot with unturned loads.

 

[David Milisock]

Theoretical, much more so than factual. Anecdotal at best.

I'm interested in the validity of the Hornady 52,000 PSI data, I want an AR style rifle for specific tasks and the medium pressure longevity is important to me.

AR hunting and controlled timed rapid fire without rapid throat erosion is important. The 52,000 PSI is an important aspect of that.

My Colt AR 15 A2 is a fabulous rifle, I simply need an AR that moves to the same cross application ballistics that 24 caliber bolt guns do for 22 caliber bolt guns.

I've been a 24 caliber shooter for nearly 40 years and have done and seen game taken with 24 caliber rifles people would not believe, I want a quality 24 caliber self loader without the short coming of high pressures. I'll leave the super customization to my Remington 700 in 6MM Remington.