In June 2020, Hornady introduced the 6mm ARC, a new SAAMI cartridge optimized for AR-platform rifles*. The new 6mm ARC is basically a 6.5 Grendel necked down to 6mm, with the shoulder moved back around .030″. That pushed-back shoulder does reduce case capacity (and velocity), but we assume Hornady did that to create a shorter, proprietary chamber so people could not simply neck-down Lapua 6.5 Grendel brass, as has been done for years with Robert Whitley’s outstanding 6mm AR wildcat.
What Is the 6mm ARC Cartridge? The 6mm ARC cartridge is a new SAAMI-spec cartridge based on the 6.5 Grendel case necked down for 6mm bullets, with the shoulder moved back 0.030. Yes it is designed to run in AR15-platform rifles. You’ll need a new barrel, bolt, and mags. If you already have an AR chambered in 6.5 Grendel, the ONLY thing you need to change is the barrel. Everything else — bolt, magazines, gas system – is compatible with 6mm ARC.
▶ Official SAAMI Cartridge (not wildcat)
▶ Fits standard AR15-platform rifles
▶ Fits Short/Mini action bolt rifles
▶ Efficient short, fat case design
▶ 30-degree case shoulder
What Do I Need To Shoot the 6mm ARC?
Faxon and Ballistic Advantage are already producing barrels, with more manufacturers sure to follow. All the other required components are already on the market for 6.5 Grendel rifles. Aero Precision already offers complete 6mm ARC uppers.
If you’re converting a standard 5.56×45 mm (.223 Rem) AR15 upper to shoot 6mm ARC, you’ll need a 6mm barrel, a Type II 6.5 Grendel bolt carrier group, and new magazines. Some folks have suggested standard AR mags will work, but trust us, you want the magazines that have been designed for 6.5 Grendel. Most of the hardware you need is available at Brownells.
* While 6-6.5 Grendel shooters are known to run stout pressures, the new 6mm ARC cartridge has a relatively moderate Maximum Average Pressure (MAP) rating of 52,000 psi according to the official SAAMI specifications. For a variety of reasons, is wise to keep pressures in a semi-auto rifle moderate. Don’t chase the velocities you might get in a bolt-action gun.
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In June 2020, Hornady introduced the 6mm ARC, a new SAAMI cartridge optimized for AR-platform rifles*. The new 6mm ARC is basically a 6.5 Grendel necked down to 6mm, with the shoulder moved back around .030″. That pushed-back shoulder does reduce case capacity (and velocity), but we assume Hornady did that to create a shorter, proprietary chamber so people could not simply neck-down Lapua 6.5 Grendel brass, as has been done for years with Robert Whitley’s outstanding 6mm AR wildcat.
What Is the 6mm ARC Cartridge? The 6mm ARC cartridge is a new SAAMI-spec cartridge based on the 6.5 Grendel case necked down for 6mm bullets, with the shoulder moved back 0.030. Yes it is designed to run in AR15-platform rifles. You’ll need a new barrel, bolt, and mags. If you already have an AR chambered in 6.5 Grendel, the ONLY thing you need to change is the barrel. Everything else — bolt, magazines, gas system – is compatible with 6mm ARC.
▶ Official SAAMI Cartridge (not wildcat)
▶ Fits standard AR15-platform rifles
▶ Fits Short/Mini action bolt rifles
▶ Efficient short, fat case design
▶ 30-degree case shoulder
What Do I Need To Shoot the 6mm ARC?
Faxon and Ballistic Advantage are already producing barrels, with more manufacturers sure to follow. All the other required components are already on the market for 6.5 Grendel rifles. Aero Precision already offers complete 6mm ARC uppers.
If you’re converting a standard 5.56×45 mm (.223 Rem) AR15 upper to shoot 6mm ARC, you’ll need a 6mm barrel, a Type II 6.5 Grendel bolt carrier group, and new magazines. Some folks have suggested standard AR mags will work, but trust us, you want the magazines that have been designed for 6.5 Grendel. Most of the hardware you need is available at Brownells.
* While 6-6.5 Grendel shooters are known to run stout pressures, the new 6mm ARC cartridge has a relatively moderate Maximum Average Pressure (MAP) rating of 52,000 psi according to the official SAAMI specifications. For a variety of reasons, is wise to keep pressures in a semi-auto rifle moderate. Don’t chase the velocities you might get in a bolt-action gun.
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In June 2020, Hornady introduced the 6mm ARC, a new SAAMI cartridge optimized for AR-platform rifles*. The new 6mm ARC is basically a 6.5 Grendel necked down to 6mm, with the shoulder moved back around .030″. That pushed-back shoulder does reduce case capacity (and velocity), but we assume Hornady did that to create a shorter, proprietary chamber so people could not simply neck-down Lapua 6.5 Grendel brass, as has been done for years with Robert Whitley’s outstanding 6mm AR wildcat.
What Is the 6mm ARC Cartridge? The 6mm ARC cartridge is a new SAAMI-spec cartridge based on the 6.5 Grendel case necked down for 6mm bullets, with the shoulder moved back 0.030. Yes it is designed to run in AR15-platform rifles. You’ll need a new barrel, bolt, and mags. If you already have an AR chambered in 6.5 Grendel, the ONLY thing you need to change is the barrel. Everything else — bolt, magazines, gas system – is compatible with 6mm ARC.
▶ Official SAAMI Cartridge (not wildcat)
▶ Fits standard AR15-platform rifles
▶ Fits Short/Mini action bolt rifles
▶ Efficient short, fat case design
▶ 30-degree case shoulder
What Do I Need To Shoot the 6mm ARC?
Faxon and Ballistic Advantage are already producing barrels, with more manufacturers sure to follow. All the other required components are already on the market for 6.5 Grendel rifles. Aero Precision already offers complete 6mm ARC uppers.
If you’re converting a standard 5.56×45 mm (.223 Rem) AR15 upper to shoot 6mm ARC, you’ll need a 6mm barrel, a Type II 6.5 Grendel bolt carrier group, and new magazines. Some folks have suggested standard AR mags will work, but trust us, you want the magazines that have been designed for 6.5 Grendel. All the hardware you need is currently available at Brownells.
* While 6-6.5 Grendel shooters are known to run stout pressures, the new 6mm ARC cartridge has a relatively moderate Maximum Average Pressure (MAP) rating of 52,000 psi according to the official SAAMI specifications. For a variety of reasons, is wise to keep pressures in a semi-auto rifle moderate. Don’t chase the velocities you might get in a bolt-action gun.
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In June 2020, Hornady introduced the 6mm ARC, a new SAAMI cartridge optimized for AR-platform rifles*. The new 6mm ARC is basically a 6.5 Grendel necked down to 6mm, with the shoulder moved back around .030″. That pushed-back shoulder does reduce case capacity (and velocity), but we assume Hornady did that to create a shorter, proprietary chamber so people could not simply neck-down Lapua 6.5 Grendel brass, as has been done for years with Robert Whitley’s outstanding 6mm AR wildcat.
What Is the 6mm ARC Cartridge? The 6mm ARC cartridge is a new SAAMI-spec cartridge based on the 6.5 Grendel case necked down for 6mm bullets, with the shoulder moved back 0.030. Yes it is designed to run in AR15-platform rifles. You’ll need a new barrel, bolt, and mags. If you already have an AR chambered in 6.5 Grendel, the ONLY thing you need to change is the barrel. Everything else — bolt, magazines, gas system – is compatible with 6mm ARC.
▶ Official SAAMI Cartridge (not wildcat)
▶ Fits standard AR15-platform rifles
▶ Fits Short/Mini action bolt rifles
▶ Efficient short, fat case design
▶ 30-degree case shoulder
What Do I Need To Shoot the 6mm ARC?
Faxon and Ballistic Advantage are already producing barrels, with more manufacturers sure to follow. All the other required components are already on the market for 6.5 Grendel rifles. Aero Precision already offers complete 6mm ARC uppers.
If you’re converting a standard 5.56×45 mm (.223 Rem) AR15 upper to shoot 6mm ARC, you’ll need a 6mm barrel, a Type II 6.5 Grendel bolt carrier group, and new magazines. Some folks have suggested standard AR mags will work, but trust us, you want the magazines that have been designed for 6.5 Grendel. All the hardware you need is currently available at Brownells. And Brownells also sells complete Aero Precision 6mm ARC Uppers, starting at $749.99.
* While 6-6.5 Grendel shooters are known to run stout pressures, the new 6mm ARC cartridge has a relatively moderate Maximum Average Pressure (MAP) rating of 52,000 psi according to the official SAAMI specifications. For a variety of reasons, is wise to keep pressures in a semi-auto rifle moderate. Don’t chase the velocities you might get in a bolt-action gun.
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A decade-and-a-half ago, our friend Robert Whitley created the 6mmAR, a wildcat based on the 6.5 Grendel necked down to 6mm. The great thing about the 6mmAR cartridge is that is offers superb accuracy with high-BC 6mm bullets, while working in AR15-platform rifles. The 6mmAR feeds and functions using 6.8 SPC or 6.5 Grendel AR magazines. We provided a full report on the 6mmAR way back in 2006.
Hornady Introduces the 6mm ARC Cartridge for AR15 Platform
Now Hornady, with no apparent credit to Whitley’s 6mmAR, has announced a copycat cartridge, which it calls the 6mm ARC (Advanced Rifle Cartridge). Hornady will offer loaded 6mm ARC ammunition and also cartridge brass. In addition Hornady will produce reloading dies, shell plates, modified case, and other accessories. SEE 6mm ARC SAAMI Specification.
Hornady states: “Hornady® is proud to introduce the new 6mm ARC (Advanced Rifle Cartridge). Recently tested, selected and fielded by a specialized group within the U.S. Department of Defense for its multipurpose combat rifle program, the 6mm ARC is a truly versatile cartridge that maximizes the potential of the AR-15 platform.” Hornady Ballistician Jayden Quinlan adds: “We subsequently modeled and tested a variety of designs in different calibers until we were able to produce the most flexible cartridge possible within the limits of the AR-15 system. The 6mm ARC does much of what larger cartridges can and everything that smaller cartridges can’t.”
NOTE: The rim diameter (0.441) and thickness of the 6mm ARC appear to be identical to the 6.5 Grendel as illustrated on page 40 of the SAAMI Industry Standards. From all we can tell, in other dimensions as well, this is a very close copy of Robert Whitley’s 6mmAR, a 6-6.5 Grendel wildcat. For example, 6mm ARC case length is 1.49″ vs. 1.51″ for 6mmAR. The Guns&Ammo report on the 6mm ARC states: “The 6mm ARC is based on the 6.5 Grendel. Hornady started by necking down the cartridge to 6mm and then pulled the shoulder back closer to the case head .030-inch. … Hornady also set maximum chamber pressure low at 52,000 pounds PSI. This slows extraction and makes it much less violent, prolonging the bolt’s life.”
There is an extensive discussion of the 6mm ARC on the Gun University website. You’ll find dimensions and performance numbers for the new cartridge along with videos showing the 6mm ARC cartridge tested in a Barrett AR-type REC7 DI rifle. Read Gun University Story HERE.
More Velocity Than a .223 Rem, Less Weight and Recoil Than a .308 Win
Hornady says the 6mm ARC “delivers substantially better ballistics than the [.223 Rem] and offers a much lighter gun/ammo system with 30% less weight than the AR-10/308 Win system. This new cartridge delivers less felt recoil than larger short-action-based cartridges, yet takes full advantage of modern, heavy-for-caliber 6mm bullets that provide excellent accuracy and ballistic performance at extended range.”
You can see a ballistic comparison between 6mm ARC, .223 Remington, and .308 Winchester in this Hornady chart:
Notably, a large number of AR gun-makers have committed to producing rifles or uppers chambered for the 6mm ARC. Some of these are: Adams Arms, Barrett, Brownells, Christensen Arms, GAP, Howa, Mossberg, Noveske, Odin Works, Radical Firearms, and Wilson Combat. In addition Geissele and CMC Triggers are listed as suppliers on Hornady’s 6mm ARC webpage.
The Proven Lapua Brass Alternative — 6mmAR and 6mmAR Turbo
If you want 6mm ARC performance in your AR15 with superior case life and consistency, consider doing the 6mmAR or 6mmAR Turbo instead. These are proven designs with winning records in high power competition. Redding produces dies, and complete uppers (with proper Grendel-sized bolt face) are available from 6mmAR.com.
Superior Accuracy with Impressive Ballistics
Robert explained to us: “Lapua’s Grendel brass is of exceptional quality, very hard at the back, with a well-annealed neck area, and it has a small primer pocket and small flash hole. When you start with this kind of brass, you have all the ingredients of great accuracy. Necked down to 6mm, it is like a long-bodied 6 PPC, that feeds and shoots well out of an AR-15. This gives you a case that can handle high pressures, with the accuracy pedigree of the 6PPC. It doesn’t get much better than that.”
To create the 6mmAR simply neck-down parent Lapua 6.5 Grendel brass, which has small primer pocket and small flash hole. Case capacity is 36.0 grains of water, filled to over-flowing. (For comparison the 6mm PPC holds about 32.5 grains). To prepare cases, simply run the 6.5 Grendel brass through a Redding 6mmAR full-length sizing die. The FL die’s internal neck diameter (0.267″) is spec’d to deliver about .004″ tension on the bullet–just right for a gas gun. Redding’s 6mmAR dies were built to Whitley’s specs and closely match the actual dimensions of the Lapua brass. This video from Robert Whitley shows how to form 6mmAR cases from Lapua 6.5 Grendel brass.
Velocities: With the recommended medium-burn-rate powders, bullets in the 85- to 90-grain range can be pushed to the 2800 to 2850 fps range. In most 6mmAR rifles, the 105- to 107-grain bullets can be driven to velocities of 2700 to 2750 fps in a 24″ or longer barrel. More speed is possible, but don’t count on it with the average barrel. If you want more speed the 6mmAR Turbo offers additional case capacity. This allows the 6mmAR Turbo to drive the heavier 6mm bullets at velocities close to 6mmBR Norma cartridge levels.
Magazines: The correct magazines are the key to a reliable, competition-worthy system. For best results with the 6mmAR or 6mmAR Turbo, use special 6.8 SPC mags or Alexander Arms Grendel mags. Those specialized mags will provide better reliability than standard .223 Rem/5.56 mags for the AR15.
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In a preceding story, we revealed the 6mm ARC cartridge from Hornady. This is VERY similar to the 6mmAR cartridge based on the 6.5 Grendel, which was developed by Robert Whitley a decade and a half ago. Hornady developed the 6mm ARC is cooperation with the U.S. Department of Defense (DOD), and now the first run of 6mm ARC black rifles is heading to DOD testers. Barrett Firearms Manufacturing, Inc. (“Barrett”) has announced that it has partnered with Hornady to chamber the REC7 rifle series in the new 6mm ARC offering. The first production run of these rifles has been delivered to the U.S. DOD, fulfilling a contract that was awarded to Barrett late last year. READ MORE HERE.
Barrett states: “The combination of the proven REC7 DI gas system and the potent 6mm ARC cartridge creates a product that is consistently accurate and easy to shoot. Carrying 24 rounds in the magazine, the 108 gr. ELD Match bullets leave the 18″ Proof Research match grade stainless steel or carbon fiber barrel at over 2630 fps. This easily gives the rifle supersonic capabilities past 1000 yards.”
In this video Ryan Cleckner (Former U.S. Army Ranger) tests the Barrett REC7 chambered in 6mm ARC:
TECH TIP by Robert Whitley,AR-X Enterprises LLC
Over the years, while working with various AR-15 cartridges that require a larger bolt-face bolt (i.e. bigger than a 5.56 NATO/.223 Rem bolt-face, like those cartridges that use a 6.8 SPC bolt or the bolt face suitable for the 6.5 Grendel-based cartridges), I have found that there is an increased potential for a certain type of jam if a modification to the standard “Mil-Spec”, square-edged ejector is not made.
The original AR-15 square-edged ejector design was made for a much smaller-diameter bolt face and the smaller diameter 5.56 NATO/.223 Rem case, and it works perfectly in that application. However, as people have adapted the AR-15 platform to shoot bigger cartridges, some parts have been modified to accept the larger cartridges (i.e. bigger bolt-face bolts for the 6.8 SPC and the 6.5 Grendel, and different extractors), yet other parts have been all but ignored. One of these “ignored” parts has been the ejector. Most of the larger-bolt-face AR-15 bolts still use the standard “Mil-Spec”, square-edged 5.56 NATO/.223 Rem. ejector. That’s the problem. But there is a simple, reliable fix!
Chamfering AR Ejector for Improved Reliablity with 6mm, 6.5mm and 6.8mm Cartridges
With the larger bolt face and the larger-diameter AR cases, the old-style “Mil-Spec” ejector can cause infrequent but still annoying jams if the ejector is not modified. The jam can occur when a cartridge case feeds up and out of the right side of the magazine, and as it does so, the back of the case must slide across the bolt face and sideways over top of the ejector if it is to center up to the chamber and feed in. If the side of the case catches on the sharp-edged ejector you can get a jam. (See picture above).
Fortunately there is an easy fix for this. One way is to take the ejector out and spin it in a lathe or cordless drill and machine or grind it and round or chamfer the sharp edge. (See picture of rounded ejector next to square edged ejector).
Quick Fix Alternative — Bevel Your Ejector
Another “quick fix” is to leave the ejector in the bolt and chamfer the sharp edge with something like a Dremel tool. (See picture). This fix is easy to do and permanently resolves this potential feeding jam issue. There are no downsides to this modification if done right and I would recommend this modification for the ejectors in all larger bolt-face AR-15 bolts.
This gunsmithing tip provided by Robert Whitley of AR-X Enterprises LLC, 199 North Broad Street, Doylestown, PA 18901. Phone: (215) 348-8789. Website: 6mmAR.com.
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Robert Whitley of AR-X Enterprises, LLC builds match-grade uppers for AR-platform rifles. Many of Robert’s favorite chamberings are based on the 6.5 Grendel case necked-down to 6mm. Until 2011, Lapua was the only source for 6.5 Grendel brass. As you’d expect, Lapua’s Grendel brass is truly excellent, but it is also pricey, and sometimes hard to find. Now Hornady is producing USA-made 6.5 Grendel brass. Robert Whitley has worked with the Hornady 6.5 Grendel brass for over a year now and he is able to assess its performance compared to the original Lapua version. Writing in our Shooters’ Forum, Robert reveals: “It’s decent brass but hot loads will loosen the primer pockets fast. With moderate loads you will get good case life and service from the brass and it can deliver excellent accuracy as well. Not Lapua but not bad either.”
Robert reports: “I was able to get my hands on some of Hornady’s 6.5 Grendel brass. My big question was how it would measure up, particularly the loaded necks, and whether it would be compatible with our existing 6mmAR and Turbo 40 die sets. As it turns out, this brass looks like a perfect fit for our existing die sets (and obviously 6.5 Grendel die sets too). Accordingly, folks with existing die sets will be able to use the Hornady brass without any issues.” However, as the loaded neck on the Hornady brass is .001″ (one-thousandth) slimmer than Lapua brass, you may want to try a smaller bushing when sizing Hornady Grendel brass.
The Hornady 6.5 Grendel brass has a LARGE Flash Hole, about .078″ versus .0591″ for Lapua brass. Dimensionally, the biggest difference is the shoulder diameter, with the Hornady brass measuring 0.428″ vs. 0.424″ for the Lapua brass. The Hornady is actually a better fit for 6mmAR chambers which are about 0.432″ at the shoulder. Interestingly, case H20 capacity is virtually identical. Water capacity of new, unfired Hornady 6.5 Grendel brass is 35.1 grains, while new, unfired Lapua Grendel brass holds 35.0 grains of H20. Both brands of Grendel brass increase to about 36.0 grains H20 capacity after firing and full-length sizing.
Here are some of the particulars of the Hornady cases:
Hornady 6.5 Grendel Brass
Lapua 6.5 Grendel Brass
Flash hole diameter: ~ .078″
OAL of brass: Average 1.515″
Weight of cases: 111.7 to 113.0 grains
Web diameter, unfired: 0.4375″
Shoulder diameter, unfired: 0.428″
Loaded neck diameter: 0.2895″
6mmAR loaded neck: 0.270″
Flash hole diameter: 1.5mm (0.0591″)
OAL of brass: Average 1.515″
Weight of cases: 111.0 to 112.5 grains
Web diameter, unfired: 0.4385″
Shoulder diameter, unfired: 0.424″
Loaded neck diameter: 0.290″
6mmAR loaded neck: 0.271″
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Dispelling Some 6mm BRX Myths, by Robert Whitley
I recently had an interesting and fact-filled conversation with Bob Crone (the inventor of the 6mm BRX) about his BRX. Bob actually called me because he was concerned that there are a lot of myths and about his 6mm BRX and he wanted to “set the record straight” on a few things. Here’s my summary of some topics we discussed:
Myth One:Bob made his 6mm BRX chamber by running a 6mm BR reamer in too deep.
Truth: There is no truth to Myth One. Bob said he specifically designed the BRX reamer and had it made with the head space he specified and a neck length appropriate for his newly designed 6mm BRX wildcat cartridge.
Myth Two:In the course of making up his 6mm BRX, Bob had a version with a .120″ longer head space than a 6mm BR Norma.
Truth: Bob was clear that his original design for the 6mm BRX always had a .100″ longer head space than a 6mm BR and that he never deviated from that. Right after Bob started working with his 6mm BRX, Bill Shehane came forward with a 6mm BRX version he made up that had a .120″ longer head space, and thus the confusion started. In truth, the original 6mm BRX always was (and still is) a chambering with a head space .100″ longer than a 6mm BR Norma.
Myth Three:The BRX was originally set up for 105-107gr bullets.
Truth: Bob said he set up his original reamer with a zero freebore and he has his gunsmith use a throater to throat whatever chamber was being made to where he wanted a particular bullet to touch the lands. Bob said he originally started with a 1:10″ twist barrel and shot a bunch of the 95gr VLDs and also 87gr bullets. Later he used an 8.5″ twist barrel for the heavier bullets.
Myth Four:The original BRX was set up also for use with Remington BR Brass.
Truth: Bob said he always used Lapua brass. He originally was an avid reader of Precision Shooting magazine and saw that the .262″ neck-turn neck was working real well with the 6 PPC and 6mm BR bench rest shooters and he decided to try to go with something that was already working well for 6mm shooters. He says his original reamer had a .262″ neck, but he also has a .272″ “no-neck-turn” version that he used on some of his rifles. He said the Lapua brass was so good and consistent, that for some rifles he just “didn’t want to mess with it” so he went with the “no-neck-turn” .272″ neck too.
Myth Five:The “false shoulder” method was used to make brass.
Truth: Bob said he never used the “false shoulder” method to make 6mm BRX brass, he just loaded 6mm BR brass with a bullet well into the lands of the BRX chamber and fire-formed brass that way. He noted that the accuracy with some of the fire-forming rounds was “outrageous” (i.e. outrageously good that is), so much so, they were shot in matches sometimes.
Myth Six:The 6mm BRX has a blown-out or straightened out body.
Truth: Bob said he kept the basic body taper of the 6mm BR cartridge. He wanted the cartridge to be an easy and inexpensive (but high performance) 6mm wildcat, and the original design was (and still is) set up to use readily available 6mm BR dies (up off the shell holder about .100″ from the normal position).
Author’s note: It was a most enjoyable experience to talk to Bob Crone. He is not only still passionate about his 6mm BRX, but he has a commanding knowledge of it as well.
Reamer Print provided by AR-X Enterprises LLC, www.6mmAR.com.
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Starting June 1, 2011, Satern Custom Machining and Liberty Barrels will no longer produce 6.5 Grendel and .50 Beowolf barrels, as both barrel makers declined to agree to a new, more onerous licensing agreement with Alexander Arms. Satern had produced cut-rifled 6.5 Grendel and Beowolf barrels for the past five (5) years under license to Alexander, while Liberty crafted button-rifled Grendel and Beowolf barrels under license. It may be hard to replace Satern, a highly regarded cut-rifled barrel-maker, as other top-quality barrel makers may likewise reject Alexander Arms’ licensing demands.
Alexander Arms’ Policies Have Impeded 6.5 Grendel Development
Radford, VA-based Alexander Arms continues to paint itself in a corner with respect to the 6.5 Grendel cartridge. Alexander Arms (AA) has aggressively protected the 6.5 Grendel cartridge design, threatening legal action against companies that planned to sell 6.5 Grendel AR uppers, chamber barrels for this cartridge, or even merely offer 6.5 Grendel reamers. One can understand Alexander’s desire to protect its design rights, but many industry experts believe Alexander Arms has gone overboard. Many gun builders and parts markers view the 6.5 Grendel cartridge as a “lawsuit in the making” so they stay away from it. As a result, this accurate and efficient cartridge is not growing in popularity as rapidly as it might otherwise. Alexander Arms has simply made it too difficult to comply with its demands, so many gun makers have decided that they would rather build anything other than a 6.5 Grendel.
Ironically, many shooters have chosen to build guns based on 6.5 Grendel parent brass, but necked down to 6mm. Robert Whitley pioneered a very effective 6mm-6.5 Grendel wildcat he calls the 6mmAR. It shoots great in AR platform rifles, as does a higher-capacity derivative, the 6mmAR “Turbo”. To learn more, visit 6mmAR.com.
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A full-time lawyer, Robert Whitley is also an avid prone, Highpower, and cross-course shooter. Over the past two years, Robert has pioneered a cartridge for these disciplines (as well as long-range varminting) based on the 6.5mm Grendel necked down to 6mm. Robert calls the cartridge the “6mmAR”.
The 6mmAR cartridge has performed very well in AR-based comp guns, both with shorter bullets loaded to mag length, and 105-107gr bullets single-loaded. Robert has complete info about the 6mmAR on his website, 6mmAR.com, including recommended loads and field test results. Robert has also developed a higher capacity version, with the shoulder blown forward and a shorter neck. He calls this the 6mmAR Turbo. This round can drive 105-108gr bullets in the 2850-2900 range, and it has proven very accurate. It is one of the best options for AR-platform space-gun shooters.
Though making 6mmAR brass from 6.5 Grendel brass is quite straight-forward and simple, Robert receives many questions about forming the brass, so he recently prepared a video. The two-minute video shows how quick and easy the brass-forming process can be:
Robert also builds and sells products for across-the-course shooters, including complete AR uppers, chambered in a variety of calibers, including 6mmAR. Jerry Tierney, 2005 NBRSA 1000-yard champion, has a 6mmAR upper built by Robert and Jerry says the upper “Absolutely shoots great. I love it.” In the past few months, Jerry has broken many NRA senior records with his new 6mmAR-chambered rifle.
The 6mm ARC cartridge is a new SAAMI-spec cartridge based on the 6.5 Grendel case necked down for 6mm bullets, with the shoulder moved back 0.030. Yes it is designed to run in AR15-platform rifles. You’ll need a new barrel, bolt, and mags. If you already have an AR chambered in 6.5 Grendel, the ONLY thing you need to change is the barrel. Everything else — bolt, magazines, gas system – is compatible with 6mm ARC. 
Dispelling Some 6mm BRX Myths, by 
Truth: Bob was clear that his original design for the 6mm BRX always had a .100″ longer head space than a 6mm BR and that he never deviated from that. Right after Bob started working with his 6mm BRX, Bill Shehane came forward with a 6mm BRX version he made up that had a .120″ longer head space, and thus the confusion started. In truth, the original 6mm BRX always was (and still is) a chambering with a head space .100″ longer than a 6mm BR Norma.
Myth Five: The “false shoulder” method was used to make brass.
