Grendel-Based 6mmAR for AR15s

The classic AR-15 “Black Rifle” remains extremely popular among High Power competitors. But its standard .223 chambering leaves much to be desired as a long-range round. Robert Whitley has come up with a great new chambering for the AR-15 that offers dramatically better long-range performance in the wind. Robert’s cartridge, dubbed the “6mmAR”, is basically a necked-down 6.5 Grendel. But Robert has done more than just tweak the Grendel. He’s put together a complete package that includes complete uppers, custom dies, and reliable magazines. Put that all together with a .556 BC bullet running 2750 fps, and you have a winning combination.


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6mmAR: AR-10 Performance from an AR-15
by Robert Whitley, www.6mmAR.com

For years I shot High Power rifle competition with a .223, and while I did very well with it, I always found it hard to do well at 600 yards, particularly when it was windy. I found myself dreading the 600-yard line on those windy days.

It seemed like there should have been a decent 6mm cartridge for the AR-15, but there never seemed to be one that was easy to do and worked well. As a consequence, I switched over to shooting AR-10s, mostly in 6mm/22-250 and 243 Win. I love shooting the AR-10s, particularly at the 300 and 600 yard lines, but AR-10s are big, heavy and expensive. I wanted something that retained the advantages of the AR-15 platform (less weight, lower cost), but with ballistics rivaling the AR-10. I knew the 105 to 107 grain 6mm bullets could make shooting the longer lines more enjoyable because if you miss a wind call or change up, you pay a price, but nowhere near the penalty you’d get shooting a .223.

A while ago I did an AR-15 High Power rifle space gun project with Alexander Arms, chambered in the 6.5 Grendel cartridge. The 6.5 Grendel emulates the 6mm PPC cartridge, the most accurate cartridge ever developed. Consider it a 6.5 PPC with more case capacity and a slightly shorter neck. While the 6.5 Grendel is a great cartridge that performs well as a High Power rifle cartridge, all the time I worked with it I kept thinking, “this thing is just begging to be a 6mm”. Ultimately I couldn’t resist, and the lure of trying to get AR-10 type performance out of an AR-15 with a 6mm bullet was just too much.

I first built a 6mmAR upper mainly by scavenging a barrel and parts from other projects and rifles. I guess it was just one of those things that just went right immediately out of the starting gate. The first upper was very accurate, it functioned and fed 100%, and I was able to propel the 6mm 105 to 107 grain bullets up in the 2700-2750 fps range. I guess I just could not believe it could really perform that well, so I built another upper thinking the first upper might have been a fluke–but the second upper was just as good as the first. I then built uppers #3 and #4, and they were both likewise successful–delivering great accuracy and impressive velocities.

Before I go further, I want to give credit and thanks to Bill Alexander at Alexander Arms. Bill has been most helpful and he has supported my work with the 6mmAR. Alexander Arms has many new products coming down the pike that can be used equally well with the 6mmAR, as well as its parent case, the 6.5 Grendel.

The 6mmAR is an easy cartridge to make, just run 6.5 Grendel brass through a 6mmAR full-length sizing die to neck it down to 6mm. (I had Redding make a special run of custom full-length dies that give .004″ tension on un-turned brass). Then load your cases and off you go to the range. The best part of it is you get to shoot Lapua brass (Alexander Arms 6.5 Grendel brass is made by Lapua–great stuff).

Plenty of other folks before me have tinkered with various 6mm wildcats for the AR. People have tried the 6 PPC in AR-15s, but there were mag feeding issues. The 6mm BR (with rebated rims) is another AR-15 option that has plenty of horsepower for the long line. 6mm BR brass is made by Lapua, Norma, and Remington but you must have someone rebate the rims in a lathe. Reloading dies are readily available. But there have been issues with feeding and reliability when using rebated 6BRs in an AR-15. Believe me if the 6mm BR was as easy to do out of an AR-15 as the 6mmAR, I’d be running a 6BR in my AR-15 right now. But, bottom line, the 6mmAR (necked-down Grendel) really is a smarter way to go, in my opinion. With the availability of Lapua 6.5 Grendel brass, plus reliable mags, excellent dies, and carefully spec’d reamers, I think the 6mmAR is truly a step forward. I wouldn’t be saying that if I didn’t have confidence in all elements of the system. I’ve literally shot thousands of rounds through multiple 6mmAR uppers. I worked through five reamer designs, refining the chamber so the brass isn’t overworked yet the cases feed reliably. The dies were carefully spec’d to match the reamer and the brass.

So, what we have is much more than just another experimental AR-15 wildcat. Here is a turnkey system that is ready to go. The system (brass, dies, mags, chamber design) works and the performance is proven. These days, I can’t wait to get to the 600 yard line because every time I get there I feel like I have a definite chance to “shoot a clean”. If you look at the wind drift charts for 600 yards with a 10 mph crosswind, you’ll see that the 6mm can shoot inside a 223 by about 10″, which is the difference between shooting a 10 and a 7–that’s huge! And for any AR-15 competitive shooters, that’s a compelling reason to move up to the 6mmAR chambering.

6mmAR Basics

Case: Derived from parent 6.5 Grendel case. Uses Lapua Brass, with small primer pocket and small flash hole. Case capacity is 36.0 grains of water, filled to over-flowing. (6mm PPC holds about 32.5 grains).

You can form the 6mmAR from 220 Russian, 7.62×39, or 6mm PPC brass. But the Lapua 6.5 Grendel brass is so good it’s crazy not to use it. Lapua Grendel brass costs about $45 per hundred. Redding’s 6mmAR dies were built to Whitley’s specs and closely match the actual dimensions of the Lapua brass. 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.

Chambering: There is no SAAMI or CIP specification. Off-the-shelf uppers are based on Whitley’s reamer design. This creates a chamber that matches the brass very closely but still allows proper semi-automatic feeding and extraction.

Powder Choices: A variety of powders work well with this cartridge. Whitley’s favorites are H4895, IMR 4895, Vihtavuori N140, and AA 2520.

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.

Magazines: The correct magazines are the key to a reliable, competition-worthy system. You can’t use standard .223 Rem mags for the AR15. You need 6.8 SPC mags or Grendel mags. Check out the detailed guide to magazines below.

Bolt and Fire Control System: You can’t shoot the 6mmAR with a standard AR-15 bolt. You need a special bolt sized for the larger PPC/Grendel rim diameter. However a standard AR-15 lower (and trigger group) can be used. This is one reason the 6mmAR is so attractive from a cost standpoint.

Dimensions and Mag Feeding: The 6mmAR case has the perfect OAL to operate out of an AR-15 platform with a variety of short- and long-line bullets. Mag-length loads with a variety of bullets can be seated close to the rifling and still fit in the magazines. Long-line loads with 105 to 107 grain bullets are seated long without the bearing surface of the bullet deep in the case. The 6.8 SPC brass is a good bit longer and this forces you to make compromises, if you set up the chamber for long-line loads, you have to jump many of the good mag-length bullets a whole lot, or choose some real blunt ogive bullets that give you no advantage over a .223 at 200 and 300 yards.

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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. It’s the ideal pressure vessel for a high pressure application where you are shooting large bullets out of a small case. 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. By contrast, the 6.8 SPC brass on the market right now is comparatively soft and has a large primer pocket and a large flash hole as well as a small case capacity. Comparing the two rounds (6mmAR vs. 6.8 SPC), why handicap yourself right out of the gate with anything less than the best?”

Robert believes the 6mmAR has clearly demonstrated its merit for the High Power competitor: “I’ve shot thousands of 6mmAR rounds now and I can tell you the thing is very, very accurate. Lately I have been shooting 105 VLDs (Berger and JLKs) at the long line and 2750 fps is very workable and accurate as heck. And, yes it definitely gives you an edge compared to shooting an AR-15 chambered for .223 Rem. In a 10 mph breeze, the 6mmAR with Berger 105s has about 10″ less windage at 600 yards compared to a .223 shooting the Sierra 80s. And it’s only a couple inches more than a 6BR running 2900 fps.”

Cartridge Bullet Mfg’s BC Velocity 300-yard
Windage
600-yard
Windage
1000-yard
Windage
.223 Remington 80gr Sierra MK 0.420 2750 fps 7.45″ 33.85″ 111.12″
6mmAR 105gr Berger VLD 0.556 2750 fps 5.38″ 23.16″ 73.95″
6mm BR Norma 105gr Berger VLD 0.556 2900 fps 4.93″ 21.61″ 68.43″
6.5 Grendel 123gr Lapua Scenar 0.547 2620 fps 5.95″ 26.07″ 83.34″
Ballistic calculations by PointBlank Software. Windage calculated based on 10 mph 90° crosswind, 500′ altitude, 70° F temp. Note, velocity for the 6mm BR is in a bolt-action rifle; 2800 fps might be a practical max in an AR-15.
The Complete 6mmAR System — Dies, Mags and More

Dies–Matched to the Chamber and Brass
Robert Whitley knew that for the 6mmAR to really succeed as a system and to become more than an experimental wildcat, there would have to be quality dies available. So, he worked directly with Redding to produce a matched full-length (non-bushing) die and Competition seater set. The dimensions of the FL die were blueprinted from the Lapua 6.5 Grendel brass, and the chamber reamer was likewise optimized for this brass. Unlike most full-length dies, the Redding 6mmAR FL die does NOT reduce the neck diameter too much. The die’s interior neck diameter will bring a fired case down to about 0.267″. This provides about .004″ of neck tension on unturned cases. After field testing, Robert feels this is just about ideal for this application. Ammo for semi-automatic rifles need more neck tension than one might use on a tight-chambered single-shot Benchrest gun. For those who want to experiment with greater or lesser tension, a Type ‘S’ FL die with bushings is also available from Redding. But most shooters should find that the basic FL non-bushing die suits their needs. It gives just the right amount of body and neck sizing to assure good feeding and a secure grip on the bullet.

Magazines–Reliable Choices with 5, 7, 12, and 26-round Capacities
Editor’s Note: When I started talking with Robert about his 6mmAR rifle project, one thing that really impressed me is that he worked very hard to develop an accurate AND reliable system, with no weak links in the chain. He wanted to make sure every component worked right. And this was doubly true of the magazines. He tested, modified, and tested some more. Shooting thousands of rounds, he’s found out what really works–what can be counted on to perform flawlessly match after match.

Right now there are three main magazine options for the 6mmAR cartridge. First, you can use a PRI 6.8 SPC magazine. With a green GI follower this will hold and feed 7 rounds reliably. With a trimmed MagPul follower, the PRI mag will hold and feed 5 rounds. For the High Power shooter, who is limited to 2+8 or 5+5 mag cycling during a match, the PRI is a good solution that works. Robert notes: “Here’s the modification I make to the factory Magpul follower on the 6.8 SPC mags to get them to feed 5 rounds reliably. All I do is cut the front underneath off so the front section is about .475″ long. This allows the follower to tilt down in the rear, which it needs to when you are running 6mmAR cases out of a 6.8 SPC magazine.”

For those who want a true high-capacity magazine, Alexander Arms now offers a high-quality 26-round Grendel magazine. Robert offers a cut-down version of the new 26-rounder. He chops off the bottom half, shortens the spring, and adds a new, machined floor plate. Robert explains: “In the photo, at top left, is the PRI 6.8 SPC magazine. With a modified GI Green follower it will reliably feed 7 rounds, and with a modified factory Magpul PRI follower it will reliably feed 5 rounds. In the top row center is a cut-down 26-round 6.5 Grendel magazine that reliably feeds all the rounds you can load into it (12 rounds). We cut them down because the 26-rounders are too long for most uses when a person is shooting prone or off a bipod or rest. I can cut them down to pretty much any length, but this gives a shooter a “dead-nuts” reliable magazine of 10+ capacity. At the right is the newly released Alexander Arms 6.5 Grendel 26-round magazine. They are made of hardened stainless steel and these work and feed great!”

6mmAR Match Uppers
Complete match rifle uppers (with 6mmAR bolt), ready to attach to your lower, are available from AR-X Enterprises, LLC. These feature hand-lapped Hart barrels. By the way, the Alexander Arms billet upper receiver with the side charging handle bolt carrier is now on the market, and it is outstanding. I just wrote an article with photos that featured that item for Precision Shooting Magazine (slated to be published in August or Sept of 2006). We also put one of them on a 6mmAR and the first time the customer took it to the 600 yard line (at the recent Camp Perry Memorial Day weekend regionals) he shot the highest score he ever shot at 600 yards (198/10x) and I am told that was a score that tied him for first in his class. That’s another great product Bill Alexander came out with for the 6.5 Grendel that everyone can benefit from. It is also my understanding that the competition 6.5 Grendel uppers that Arne Brennan now sells have the billet upper receivers on them. As you can probably tell, just because I am now working with a 6mm version, I still feel the 6.5 Grendel is a great cartridge for a lot of reasons.

Hardware and Smithing–Putting Together a 6mmAR Rig

It’s easy to set up an AR-15 for the 6mmAR cartridge because all rifle components (except barrel and bolt) are standard .223 AR-15 parts. Minimal modifications to the rifle are necessary to make an AR-15 function in 6mmAR trim.

If you have an existing AR-15, you can use your existing lower. You’ll need to purchase a separate 6mmAR match upper or add a new barrel and bolt assembly to your current upper. Note, the upper in the photo above has a side charging handle. We built that for a customer who uses a stock with an adjustable cheek piece that interfered with the conventional charging handle.

The barrels we typically use are Hart 1:8″ twist barrels finished at 26″. We use an aluminum free-float tube and a low-profile, clamp-on gas block that does not stress the barrel. The free-float tube includes a bipod stud (visible in the picture) for guys that may want to use a bipod for F-Class target work, general shooting, varmint hunting, etc.

The only other items you will need will be reloading dies (Redding 6mmAR match die set), magazines and brass, all of which are readily available and can be purchased from AR-X Enterprises, LLC. You cannot use the standard .223 magazines since the 6mm AR cartridge is bigger in diameter than a .223 and needs a magazine designed for it. Some brands of AR-15 magazines designed for use with the 6.8 SPC and the 6.5 Grendel cartridges work well with the 6mm AR and should be used instead of some others, to guarantee reliable feeding and function. The good news is there are readily-available magazines in the commercial marketplace that work great in a 6mmAR AR-15. The only case preparation necessary is to first run the cases through the 6mmAR FL resizing die to neck them down to 6mm before loading and firing them (no neck turning, no trimming and no special fire-forming).

To obtain match rifle uppers, die sets, brass, magazines and bullets, contact:

AR-X Enterprises, LLC (Attn: Robert Whitley)
Website: www.6mmAR.com
199 North Broad Street
Doylestown, PA 18901
(215) 348-8789

To purchase a 6mmAR rifle, or to rebuild or rebarrel your rifle to a 6mmAR, contact:

Sabreco, Inc.
2013 Voit Dr., P.O. Box 1387
Skippack, PA 19474
(610) 584-8228

Lapua 6.5 Grendel brass and Grendel magazines can also be purchased from:

Alexander Arms
US Army, Radford Arsenal
P.O. Box 1
Radford, VA 24143
Phone: (540) 639-8356

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6mmAR Load Information

Editor’s Note: The following load information was provided by Robert Whitley. Robert asked us to include this warning: “The loads below are only examples of loads that were used in other rifles with the bullets, brass, primers and seating depth noted below. Your rifle may be different than the rifles that used these loads and the loads noted below may not be appropriate to shoot in your rifle. As a reloader, it is your responsibility to work up safe loads in your rifle. As a suggestion, start by reducing the powder charge for the listed loads at least 1.5 full grains and working from there. There is also some large primer commercial brass that may be used to make brass for this cartridge, and bear in mind that a large primer will also increase pressure significantly. If large primer brass and large primers are used, powder charges should be lowered at least two full grains from the listed loads to start.”

Recommended Bullets: A wide variety of bullets from 85 to 107 grains shoot very well in 6mmAR rifles. Personally, I favor the 105gr Bergers and 105gr JLKs, but you can get excellent results with many other 6mm BTHP match bullets. You should test various brands to see what your barrel likes. (Editor’s Note: We’ve seen some 6mm barrels that have a strong preference for Berger 105s vs. Scenar 105s and vice-versa. You may also be surprised how well the shorter 75-80gr Flatbase bullets shoot out of fast twist barrels. Don’t hesitate to try the Fowler and Berger 80-grainers for 100, 200, and 300-yard work.)

Recommended Brass: While you can form 6mmAR cases from 220 Russian, 6 PPC, or 7.62×39 brass, there is no good reason to do so. The loads stated below are all for Lapua-made 6.5 Grendel brass.

Recommended Primers: Use CCI BR-4 Primers or Rem 7 1/2 BR Primers. Some other brands of primers have cup metal that appears to be thinner (or softer) and some of these other brands of primers pierce easily in an AR-15.

Thoughts on Seating Depth: I normally jump conventional bullets .010″ to .020″ for conventional bullets. With the VLDS (such as the Berger 105s and JLK 105s) I generally seat them at or in the lands .005″ to .010″. While substantial variances in bullet lots make it impossible to state exact OAL dimensions with a new 6mmAR chamber, the following are approximate OAL loadings with these bullets and are given to provide a rough idea of OALs:

Short-Course OALs 87gr Hornady BTHP (.376 B.C.) – 2.170″ OAL (.020″ jump)
90gr Berger BT (.410 B.C.) – 2.240″ OAL (.010″ jump)
Long-Line OALs 105gr Berger BT Match (.527 B.C.) – 2.250″ OAL (.010″ jump) (Recent production bullet similar to what used to be called the “LTB”.)
107gr Sierra MatchKing (.530 B.C.) – 2.325″ OAL (.010″ jump)
105gr Berger VLD (.556 B.C.) – 2.340″ OAL (.005″ in the lands)

Favorite Single Bullet “Across The Course” Load
For a single “across the course” load, a favorite with some rifles has been 27.8 grains Hodgdon’s H4895 with a 105gr Berger BT bullet (not the VLD bullet) about .010″ off the lands (about 2.250″ OAL) with a CCI BR-4 primer and necked down 6.5 Grendel (Lapua) brass. Velocity in most rifles is around 2700 to 2730 fps. Note: this load is with the new production of the Berger 105gr BT bullet, not the older 105gr LTB bullet.

Favorite Short Course and Practice Load
For the Short Course, I’ll use 27.5 to 27.8 grains of Hodgdon’s H4895 with a Hornady 87gr BTHP bullet about .020″ off the lands (about 2.170″ OAL) or a Berger 90gr BT bullet about .010″ off the lands (about 2.240″ OAL) with a CCI BR-4 primer and necked down 6.5 Grendel (Lapua) brass. Velocity of this load is around 2800 to 2850 fps in most rifles.

A Note about Moly-coated Bullets: The loadings with these powder charges were with plain (non-moly) bullets. Moly bullets will usually require approximately one-half grain more powder to achieve the same velocities. While it appears that there are some advantages associated with the use of moly bullets in the 6mm AR, because of the ongoing debates and controversy relating to the use and misuse of moly, we generally do not recommend the use of moly bullets, however, if you are going to use moly bullets, the only bullets we feel may be appropriate are the factory moly-coated Berger or Sierra bullets.


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Entire article Copyright © 2010 AccurateShooter.com | 6mmBR.com, All Rights Reserved. Images also Copyright © 2006, Robert Whitley, All Rights Reserved. No reproduction of any content without advanced permission in writing.

Topics: 6mm, 6mmAR, Highpower, High Power, DCM, Service Rifle, Long Line, Palma, Spacegun, 6mm BR, 6BR, 6BR BS, 6.5, 6.5 Grendel, 6.8 SPC, .223, AR, AR15, 6PPC, Colt, Upper, Lower, Berger, Hornady, Lilja, Krieger, Shilen, 1000 yards, IBS, NBRSA, F-Class, Varmint, Varminting, Alexander Arms, PRI, Precision Reflex Inc., Magazine, Benchrest, BR, Bench Rest, Single-shot, competition, rifle accuracy, Norma, AA 2520, Accurate Arms, Hodgdon Powder, Varget, H4895, IMR 4895, Vihtavuori, N140, N540, Berger, Lapua Scenar, Sierra, MatchKing, Competition Shooting, stocks, Robertson Composites, Kelbly’s, stainless barrel, Tubb 2000, reloading, powder, case forming, Lapua Brass, Scenar, bullets, precision.

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