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Discussion Starter · #1 · (Edited)
Hello, everyone.

Fairly new to the XCR and excited to do some shooting with what should be an interesting and fairly unique rifle.

I am intrigued by what I'm seeing on a brand new cartridge from Hornady called the 6mm ARC.


So how responsive is RobArm to new calibers? And what are the chances of getting a barrel extension and gas block if barrels are not going to be available fairly soon?

TIA


Edit--I see an outfit called Wolverine Supply has barrel extensions and gas blocks.
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I am partial to the 6mm bullets from the research I've done. Problem will be widespread adoption (as is always the problem with boutique rounds) and cost. I think it's really stupid that we keep designing things around the STANAG mag size. Just a little bigger and you'd really open up the optimized cartridge.

What's the 6 ARC like out of a short bbl....like 10"? Sounds like this is really set up to be 18" for a 1K yard gun. At 21 minutes they talk about it more...12" was the shortest...26" was longest, but they really didn't go into comparison vs. something like 6.8 SPC II which was designed for short bbls.

And what's the mag reliability....for 6.5G it's not cut and dried b/c the case is so short. Is the straight part of the case longer than 6.5G?

What mags does this use? The video didn't say. I'm assuming 6.5G.

What about belt fed? Can links be clipped on this case? That was allegedly a big reason the military didn't pick up 6.5G.

BTW, are you in Canada? If not, I'm not sure what the import rules are for getting a bbl extension from Wolverine Supply.

Just looked on Wolverine Supply's site....no joy. May have stopped selling XCR parts b/c of the ban. Not sure RA is selling extensions by themselves, but a contact to Holly is a good idea.
 

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I wonder how 6mm ARC compares:

General Purpose Combat Cartridge Revisited - Abe's Gun Cave

NOTE: I'd suggest clicking the link above since it has all the graphics and is an easier read.

General Purpose Combat Cartridge Revisited
Abe March 24, 2016 General Purpose Combat Rifle 35 Comments
6mm SAW general purpose combat cartridge

The 6mm SAW cartridge. A great general purpose combat cartridge the US military designed, but never adopted

Since writing my original General Purpose Combat Cartridge Article. I’ve done more research, more thinking and learned a LOT.

But most importantly, I’ve also shed some assumptions I didn’t realize I had.

In the original article, I only looked at bullets between 6.5mm and 7mm which weighed between 120gr and 130gr. That’s because the US military decided in the 1920s and early 1930s that was the ideal size for a military bullet.

But what if it’s not?

Remember, the military was using only full metal jacket (FMJ) bullets. Traditional FMJ bullets pretty much suck at killing things compared to modern options.

Therefore, a better designed bullet could be smaller and still be effective.

So I expanded my horizons. I tried smaller bullets and played with the numbers a little bit to find a better general purpose combat cartridge. The results floored me and made me almost giddy for the better part of a day.

Click Here for some nifty drop-down text with a summery of the previous article
General Purpose Combat Cartridge Round 2

Here’s the updated/expanded chart. (These values were created using a G7 BC assuming a form Factor of 1.0. With a form factor of 1.0, then G7 BC = sectional density. The bullets all have a Sectional Density around .261. +/- .001)
Bullet Diameter Bullet weight Recoil (ft-lbs) Muzzle Velocity 800y Velocity 800y Drop 800y Drift Muzzle Energy 800y Energy 800y HITS
5.56mm 92 10.1 3180 1866 -133.7 40.7 2066 711 450
6mm 108 8.5 2830 1600 -176.2 49.4 1920 614 450
.257 120 10.6 2837 1602 -176.1 49.5 2144 684 450
6.5mm 127 9.4 2510 1362 -235.1 60.4 1776 523 450
6.8mm 140 9.3 2330 1232 -279.6 68.0 1688 472 450

First, a 92gr 5.56 bullet @ 3180 fps would burn out a barrel so fast it would never work as a general purpose combat cartridge.

But look at the 6mm bullet.

It has the least drop, least wind drift, and the least recoil while still being more lethal than our current 147gr 308 load.

Did I mention almost HALF the recoil?

Compared directly to the 5.56 (with 20″ barrel), the 308, and our previous 6.5mm cartridge, we get this:
Cartridge Bullet weight Recoil (ft-lbs) Muzzle Velocity 800y Velocity 800y drop 800y drift Muzzle Energy 800y Energy 800y HITS
M855 62gr 3.8 3112 1105 -213.9 94.1 1333 168 121
6mm GPCC 108 8.5 2830 1600 -176.2 49.4 1920 614 450
6.5mm GPCC 130 10.5 2600 1450 -213 55.3 1951 607 501
M80 Ball 147gr 15.8 2800 1277 -215.2 72.8 2559 532 415

Our 6mm bullet easily beats both the 308 and 5.56 in Every Single Category except muzzle energy, where it only loses to the 308.

Compared to all of them, the 6mm option has 3 feet less drop.

It beats the 5.56 in wind drift by almost 4 feet and the 308 by almost 2 feet, though it beats the 6.5mm option by only 6 inches.

General Purpose Combat Cartridge Trajectory Chart

Compared to our previous 6.5mm general purpose combat cartridge, the 6mm option wins in drop, drift and most importantly recoil. However, the 6mm loses slightly – about 11% – in long range lethality.

However, the 6mm option has a higher hit probability due to less drop and wind drift.

That fact alone makes me pick the 6mm over the 6.5 because Shot placement is King.

The most important factor in stopping an enemy attacker is shot placement. Less drop and drift make shot placement easier. Many people (including myself) would argue that the 6mm bullet is more lethal because shot placement (the most important factor) is easier.

And you can’t miss fast enough to win a gunfight.

The 6mm has less recoil which means faster follow up shots in close quarters combat. Plus, higher velocity which is a huge help against armor. (speed defeats armor better than high mass.)

I don’t think a theoretical 11% increase in lethal potential is enough to justify a very real 23% increase in recoil, a 21% increase in drop, and 12% increase in wind drift….

Especially when the 6mm bullet is lethal enough.

The 6mm bullet is already more lethal than the 308 at 800 yards if you go by the numbers.

To give you a sense of scale, 800 yards is over 6 1/2 football fields away (including the end zone) and our little 6mm bullet could kill an enemy soldier at that range.
A Note On Bullet Lethality

To be clear, I don’t think a 108 grain 6mm bullet at 1600 FPS (800 yard velocity) will hit an enemy soldier like the Hammer of Thor.

It simply can’t.

But it can hit like a .357 Magnum hits at point blank range, and feel free to double check me on that.

A lightweight ~110 grain .357 magnum can reach in 1500-1650 FPS range. Some will point out that the .357 magnum is a much bigger bullet so it will do more damage. That’s (sort of) right in theory, but wrong in execution.

What matters is what a bullet does and most importantly, WHERE it hits.

A properly constructed 6mm bullet can do everything a .357 magnum bullet of equal weight and velocity can do.

Emphasis on “Properly constructed”.

I’ll talk more about proper bullet construction in a minute. In the meantime, do we agree that a .357 Magnum at point-blank range is VERY deadly?

We do?

Good.

Moving on.
Back to the Future

I’m not the first person to realize the potential of a 6mm general purpose combat cartridge.

The United States Navy is.

6mm Lee NavyIn fact, it was officially adopted by the Navy in 1895 and saw service in the Spanish American War. It’s called the 6mm Lee Navy and it fired a 112 grain 6mm bullet at 2560fps.

“Contemporary medical reports of the day noted the 112-grain (0.26 oz; 7.3 g) bullet produced noticeably greater damage to tissue and bone than other military cartridges of the day when fired at full velocity (2,560 fps)”

Note: the “cartridges of it’s day” included the powerful .45-70 cartridge.

However, it didn’t do much damage unless it yawed. (turned sideways/tumbled) And since it used a round nosed bullet, it rarely yawed.

It was truly ahead of it’s time… And that’s what killed it.

The 6mm Lee Navy was invented before the modern Spitzer shape bullet and it’s round nosed bullet lost velocity very quickly. Even so, it was considered to have an effective range of 500-700 yards. (depending on who you read.)

Additionally, the metallurgy of the time couldn’t handle the small bore combined with high pressure. This was made worse because smokeless gunpowder was still quite new. The powders of the day made the barrel wear problem worse.

Ultimately, the Navy dropped the 6mm Lee Navy in 1899.

Back to the Future Pt 2

A 6mm general purpose combat cartridge was considered again by the United States in the 1970s. It was called the 6mm SAW and it fired a 105 grain 6mm bullet at 2520 FPS.

The performance was great, but the military didn’t want a 3rd cartridge in it’s inventory. Eventually, 6mm SAW project was cancelled when a new and improved 5.56 round was promised. This new round arrived as the M855. The new M855 didn’t make the 5.56 more lethal, and contributed to the 5.56/223’s reputation as both a great and awful cartridge.

The lack of technology killed a 6mm general purpose combat cartridge the first time, and money killed it the second.

Back to the Future part 3 will feature some horses, a train, and a modified Delorean.

Just kidding. ��

Now, lets talk about bullet construction.
Bullet Construction

One of the most important factors that’s ignored by the Hornady HITS calculator is the type of bullet used.

Quality Hollowpoints are best, but they’re expensive and against the Hague convention. Fortunately, there are several alternatives that are cheap enough and effective enough to be seriously considered.
M855 Cross Section Diagram

M855 Cross Section

M855 (62gr “Green Tip” Light Armor piercing)

The M855 is the current standard issue bullet to our military. It consists of a steel penetrator sitting on top of a lead core and surrounded by a copper jacket.

To be clear, I don’t think the M855 construction should be used exactly as configured because the M855 bullet has two significant problems.

Problem #1: The jacket is too strong. The M855 relies on mostly on fragmentation to deal it’s damage. However, the copper jacket is much stronger than it needs to be. This means higher velocities are needed to make it fragment which reduces effective range.

Problem #2: There is no design mechanism to ensure the bullet yaws (turns sideways/tumbles). Quite simply, the bullet relies on fragmentation, but there’s nothing in the design to ensure it fragments. This makes the M855 inconsistent at best in combat.

When it does what it’s supposed to, the M855 is amazing. However, it doesn’t do it consistently.
Mk 318 Mod 0 cross section

Mk 318 cross section.

MK318 Mod 1 SOST

The bottom half of the Mark 318 bullet is basically a solid copper slug, while the top half is a conventional Open Tip Match (OTM) Bullet. They have reasonable accuracy, fragment at high velocity, and the solid copper base penetrates through barriers with enough force to cause damage.

However, Fragmentation is VERY velocity dependent.

Out of a 14.5 barrel, the current M855 bullet only fragments well out to 60-150 yards. The MK318 Mod 1 SOST, even at near 3000 fps muzzle velocity, only fragments well out to 200-250 yards.

Because of the low effective range, I prefer a bullets where fragmentation isn’t required to be effective. There’s at least one design that does that quite well.

7N6 Cross SectionThe 7N6 bullet of the 5.45×39.

The 7N6 bullet has a hollow cavity in the front of the bullet which deforms when it hits the target. This causes the bullet to destabilize and start tumbling almost the instant it makes contact. The tumbling bullet creates significant cavitation and a suitably impressive wound channel.

Additionally, a large part of the bullet is a steel core which allows it to punch through armor with relative ease.

It doesn’t fragment like the 5.56 bullets do, so the 5.56 creates more impressive wounds at close range. However, the 7N6 wounds are effective, especially when you consider they’re being done with a 53 grain bullet.

Imagine what a 108 grain bullet could do…

Additionally, because of the air pocket in the nose, the 7N6 bullet will start tumbling almost instantly at virtually all ranges.

Since we are talking about an 800 yard combat range, being effective at ALL ranges is VERY important.

The problem with the 7N6? No Fragmentation.

While it tumbles almost instantly, it won’t fragment.

Like ever.

So Here’s my solution: Combine the M855 and 7N6 into one “super bullet” that’s still Hague convention legal.

And by that I mean keep the basic M855 design (full copper jacket, steel penetrator in front of a lead core) But add the air space in the nose of the bullet like the 7N6 to ensure the bullet will tumble and fragment.

At close range it will fragment causing a devastating wound. At long range it will tumble and still cause significant damage. Plus, the steel core penetrator will allow our soldiers to shoot through most common barriers the bad guys hide behind.

It would be an amazing bullet for our general purpose combat cartridge.

Longer bullets with thinner jackets fragment at lower velocities. A 108 grain 6mm bullet is already quite long, and will be longer with the air space and steel core.

I would make the jacket as thin as possible while still ensuring the bullet will hold together in flight. This should extend fragmentation range as far as possible.

As an alternative, the Mk 318 design could be used. That would simplify construction and possibly be cheaper on a per-round basis. Also, because it requires fewer components it might be easier to make accurate.
The Cartridge

So the next obvious question is: What cartridge should fire this 108 Grain 6mm bullet at 2830 FPS?
6mm BRX Bob Crone's Version

6mm BRX (Bob Crone’s Version)

There are two cartridges currently available to give us a working base cartridge. The 6mm BRX (Bob Crone’s version) and the 6mm Dasher.

Either can propel a 108 grain bullet to at least 2830 FPS from a sensible 20″ barrel. (I’m assuming a bullpup design to make a 20″ barrel practical)

Both cartridges use a standard .308 Winchester case diameter (.4728″) and both have an overall length of 2.44 inches. Both are almost exclusively used by the target shooting community and noted for EXTREME accuracy.

Accuracy is obviously needed for an 800 yard general purpose combat cartridge.

However, the needs of a military cartridge are different that a target shooter. First and foremost, barrel life is a consideration.

The enemy of barrel life is pressure.

In order to maintain velocity with less pressure, you need more powder and a larger case. If you move the case shoulder and neck forward a little, you can get that extra room.

Going from the 6 BR to the 6 BRX involves moving the shoulder .100 and gains about 200 FPS velocity. So another .100 should lower the pressures significantly with the same velocity. (and then lengthening the neck to at least .25 to properly hold the bullet)

The lower pressure plus modern technologies like nitride treated barrels should extend barrel life to equal or exceed the 308.

Cartridge Weight.

Looking at the weight of 6BRX brass plus our lengthened case gives us a ballpark weight of 131 grains for the case. The bullet is 108 grains, the primer is about 6 grains and the powder charge would probably be around 35 grains. (again looking at the 6 BRX numbers)

Added together, that gives our 6mm a weight of 280 grains per round. For comparison, the 5.56 M855 weighs 183 grains per round and the 308 M80 weighs 393 grains per round.

For ammo weight, that gives us: (not including magazines)

38.3 rounds/pound for 5.56
25.0 rounds/pound for our 6mm
17.8 rounds/pound for 308

To look at it another way, our soldier carry 210 rounds of 5.56 M855, or 5.49 pounds of ammo. For that same weight our soldiers could carry: (rounding up 0, 3 & 2 respectively)

210 rounds of 5.56 M855
140 rounds of our 6mm
100 rounds of 308

Because this is a low pressure cartridge, a steel case could be used. That would make it slightly lighter than a brass case. Better still, the 6mm SAW successfully used Aluminum cases which would make our 6mm cartridge really light. (only ~45 grains for the case, 194 grains for the total)

EDIT: A company called Shell Shock Technologies has created a Nickel-plated Aluminum cartridge that is supposed to withstand 65,000 PSI. Since our cartridge will probably only be ~50,000, it should work perfectly. That would make our total cartridge weight roughly 200 grains. That’s only 10% more than the current 5.56 ammo. Plus, they are cheaper to make than conventional brass cases.

But the fact remains our 6mm General purpose combat cartridge means few rounds on a per-soldier basis. Having fewer rounds isn’t exactly a desirable thing. However, I will make this argument:

I’ve talked to a lot of rank and file combat veterans from Iraq and Afghanistan to as far back as Vietnam. The vast majority say the light recoil and large amount of ammo carried with the 5.56 allowed (almost encouraged) them to “spray and pray”. And I’m not talking about suppressing fire either.

That’s both Army and Marine Corps soldiers. (yes even the Marines spray and pray)

You can’t miss fast enough to win a gunfight.

Period.

Our little general purpose combat cartridge would double the recoil and reduce the number of rounds carried. perhaps that would be incentive to help our soldiers slow down and actually aim their weapons.

To back that up, our soldiers used an estimated 250,000 bullets to kill a SINGLE enemy insurgent in Iraq and Afghanistan. Yes, that’s two hundred and fifty THOUSAND bullets for a single enemy kill.

In Vietnam where there was a mix of the M14 and M16, the ratio was one-fifth of that at 50,000 bullets per enemy kill. In WWII, it was around 20,000 bullets per enemy kill.

I partially attribute this to better marksmanship training and partially to heavier recoiling weapons. Regardless, less “spray and pray” shooting is a good thing.
A Quick Word About the Rifle and Optics

Our little 6mm cartridge would require a new rifle because it’s too long and wide for an AR15 action. I believe a bullpup design is necessary to get a barrel long enough to get the needed velocity while still being a practical overall length.

Someone would need to design that rifle, and I’d love to be the guy.

I have been designed a bullpup with CAD software for over a decade. Currently it weighs only 5 1/2 pounds empty, is completely ambidextrous, has a 20″ barrel, and is only 26 1/8″ inches long (without muzzle device.) It also has a TINY parts count, (comparable to a Glock) and can be field stripped in mere seconds with no tools.

I designed it from the ground up as a general purpose combat rifle.

As to the rifle optics.

I recommend a low powered variable scope in the 1-6 power range like the Trijicon VCOG. At 1 power, it functions much like a reflex sight. At high power you can accurately make hits out to 800 yards with a proper reticle.

In my mind, the best reticle available is the 1-6x Advanced Combined Sighting System (ACSS) reticle from Primary Arms. I would also consider the reticle in the Trijicon VCOG, though the ACSS Reticle is FAR more advanced and useful.

If I had my druthers, I would take a Trijicon VCOG with the Primary Arms 1-6 ACSS reticle (First Focal Plane of course). In my mind, that would be the perfect scope for a general purpose combat cartridge.
The “Secret Sauce”

The truth is, I’ve actually been selling our little 6mm general purpose combat cartridge short this whole article.

No, seriously.

That’s because I’ve been using a G7 Form factor of 1.0

(If you don’t know what form factors are, the chief ballistician of Berger Bullets Brian Litz wrote a wonderful article on Form factors. You can find it Here)

Short version: the better the form factor, the more aerodynamic the bullet. (Lower is better)

I’ve been using a form factor of 1.0 for our 6mm bullet. However, other military bullets have much better form factors. For example, the 7N6 bullet has a Form Factor of .920. That’s significantly better than 1.0 I’ve been using.

So if I recompute with the more aerodynamic form factor, the results look like this:
Cartridge Bullet weight Recoil (ft-lbs) Muzzle Velocity 800y Velocity 800y drop 800y drift Muzzle Energy 800y Energy 800y HITS
6mm
1.0 FF 108 8.5 2830 1600 -176.2 49.4 1920 614 450
6mm
.920 FF 108 8.5 2830 1683 -169.6 44.5 1920 679 474

Yeah, the difference isn’t mind blowing. But over 80 FPS difference at 800 yards is pretty great. Plus 6 inches less drop and 5 inches less wind drift.

Not bad for our little 6mm general purpose combat cartridge.
 

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Discussion Starter · #5 · (Edited)
This is what sets the ARC apart from most other 6.whatever AR wildcats. Military acceptance. Funny it wasn't based off the 6.8 SPC given that better mags seem to exist and that that cartridge started out as a serious possible 5.56 replacement.

I don't know enough about the differences in case capacity and the like to know why Hornady went with the Grendel case. But I'm sure Hornady knows what it is doing especially when it comes to designing target cartridges. Besides the .220 Russian and PPC have long been known to be accurate and efficient.

I always thought a 6mm Hagar would be a fun rifle but there's no real reason to pursue anything else now.

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This is what sets the ARC apart from most other 6.whatever AR wildcats. Military acceptance. Funny it wasn't based off the 6.8 SPC given that better mags seem to exist and that that cartridge started out as a serious possible 5.56 replacement.

I don't know enough about the differences in case capacity and the like to know why Hornady went with the Grendel case. But I'm sure Hornady knows what it is doing especially when it comes to designing target cartridges. Besides the .220 Russian and PPC have long been known to be accurate and efficient.

I always thought a 6mm Hagar would be a fun rifle but there's no real reason to pursue anything else now.

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The reliability of the 6.8 SPC mags is largely due to the straight walled case length. That length is much shorter in the 6.5G...which means the cartridge can 'walk' around in the magazine more easily since there's an air gap that is larger because the taper and neck are so much closer the the base.
 

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Discussion Starter · #7 ·
The reliability of the 6.8 SPC mags is largely due to the straight walled case length. That length is much shorter in the 6.5G...which means the cartridge can 'walk' around in the magazine more easily since there's an air gap that is larger because the taper and neck are so much closer the the base.
I have three rifles chambered in 6.8 SPC and am familiar with the magazines. The best are made by Barrett and/or PRI. Both are very good quality and work well. Barretts are longer and have a higher capacity. PRI are closer in size/length to USGI so may work better depending on mag pouches etc.

The less expensive mags on the market are okay for range use but don't stand up too well to abuse/dirt in my experience.

I have no experience with the 6.8 Magpul mags nor with the LWRC receivers but those are liked by their owners. I'm sure they're good stuff. Mad Dog Weapons Systems was working on a receiver set and metal magazine based off the LWRC/Magpul format. The idea was to allow for a longer overall length for some of his wildcat offerings. Not sure if he's still working on this or not.

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I looked at the six8 for a bit. The mags were available from distributors at a decent price. I thought I'd read that because of the thicker walls on the mags, you didn't an appreciable amount of extra room in the mag for an increase in length. There were several comments about wanting the PRI mags for the six8 receivers in order to go a little longer. I lost interest and haven't looked since.
 

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I have three rifles chambered in 6.8 SPC and am familiar with the magazines. The best are made by Barrett and/or PRI. Both are very good quality and work well. Barretts are longer and have a higher capacity. PRI are closer in size/length to USGI so may work better depending on mag pouches etc.

The less expensive mags on the market are okay for range use but don't stand up too well to abuse/dirt in my experience.

I have no experience with the 6.8 Magpul mags nor with the LWRC receivers but those are liked by their owners. I'm sure they're good stuff. Mad Dog Weapons Systems was working on a receiver set and metal magazine based off the LWRC/Magpul format. The idea was to allow for a longer overall length for some of his wildcat offerings. Not sure if he's still working on this or not.

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I had the LWRC and the Barretts (as well as an XCR 6.8 SPC but I never actually shot it and sold it b/c of the cost of ammo). Agreed. Good stuff....but I also had CProducts that were 100% in 6.8 and I think that had to do more so with case design/shape than any inherent 6.8 specific magazine/follower, IME of course.
 

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BTW, contacted RA today. Got an answer about bbl extensions cost:

Sean,

We can sell them for $60 for XCR-L and $70 for XCR-M. They are heat treated and the outside has to be turned down a bit after installation to maintain concentricity with barrel.



Sincerely,

RA Sales Dept.


Thinking of picking up a couple just to have on hand.
 
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