6mm Ammo, 6mm cartridge, lead
Photo by Sean Utley

The precision rifle community is continually looking for improvements. This isn’t stock racing, this is formula cars and sports prototypes. Every piece of the rifle is picked over for added performance, decreased time during use, smoother function, lower recoil and better ballistics. Which brings us to the 6mm.

Some brief history on the last 10 years of precision rifle matches is in order. Ten years ago, the 6.5 Creedmoor was the newest kid on the block. Meanwhile, the 6.5s were starting to catch on and the .308 Winchester still made up most of the field at matches. Today the .308 has its own class. Meanhile, the 6.5s are for those concerned about barrel life and the 6mm lineup is ever more crowded. Wildcats upon wildcats are more prevalent. There seems to be a new caliber of the month quite literally every month.

The 6mm Spectrum

This article’s aim is to give readers a broad-strokes introduction to the world of the 6mm competition cartridges. We’ll discuss the six 6s that comprise the majority of the performance spectrum, from the 6mm Bench Rest and its wildcats to the 6mmx47 Lapua case, Creedmoor and the granddaddy of the bunch: the .243 Winchester.

Starting from the largest and working backward gives us a benchmark of performance as well as some history. So, we’ll start at the .243 Winchester.

Good Ol’ .243

The .243 is based on a .308 Winchester parent case necked down to hold 6mm bullets. It holds about 45 grains of powder, depending on powder type, and it’s extremely flat shooting. The recoil is low, brass is easily available from many sources including Lapua, Hornady, Winchester, Remington and numerous others. Back when the 6.5 Creedmoor was just starting to show up, this cartridge was already a well-known competitor and hunter’s caliber. It is exceptional for deer hunting in open country and is highly accurate.

However, there are some significant drawbacks to the cartridge. The shoulder angle is shallow (20-degree half angle), and it has a relatively short neck. Coupled with the large case capacity, it is a well-known barrel burner. It sometimes yields competition shooters under 1,000 rounds and rarely more than 1,500 rounds of competition-grade barrel life.

But it is fast, flat, accurate and easy to load. With appropriate barrel twist rates for the modern 6mm bullets, the cartridge is extremely effective on deer-sized game past 800 yards; the same goes for target shooting to 1,200 and beyond. Load data in books tends to be pretty conservative. However, with new, high-BC bullets and the near-magical properties of powders like RL26 and N560, shooters are seeing performance that was previously unheard of without a 6mm-284 or 6mm-06 wildcat.

6mm Creedmoor

The next case is what behaves like a modernized .243 Winchester—the 6mm Creedmoor. The useful case capacities are similar and the internal ballistics are pretty similar when loaded to the same pressure and using the same powders. I’ve seen several rifles shooting 115-grain bullets over 3,200 fps with cases stuffed up on RL26. However, the barrel life was pretty short. More reasonable barrel life comes with cases shooting 105- to 110-class bullets around 3,100 fps. Lifetimes of 1,400 to 1,800 rounds are not unreasonable.

6mm Ammo, 6mm cartridge, spectrum
Photo by Sean Utley
The 6mm cartridge spectrum is varied—from the diminutive 6mm BR (far left) to the venerable .243 Winchester (far right). The performance varies but all 6mm rounds share the common properties of low recoil and excellent long-range repeatability. 

The cartridge is shorter overall than a .243, but with a sharper, 30-degree shoulder, longer neck and typically longer freebore lengths in the chamber, the powder charges are very similar as are the velocities. Excellent factory ammunition is available from several companies that are easily match grade. Hornady, Federal and Prime Ammunition all offer several flavors of match bullets that are sub-MOA capable at 1,000-yard distances.

Today’s Variants

Those are at the bigger end of the 6mm flavors—the stuff that was more popular in 2015 than in 2018 and 2020. Today, we see a massive resurgence of the venerable 6mm BR and its children. The fastest-growing caliber in the Precision Rifle Series in 2017 and 2018 was the 6mm Dasher, a wildcat based off the 6mm BR.

There are a few considerations that these little calibers really struggle with though, namely ejection and smooth feeding. The cases are very short, with trim lengths around 1.55 inches, approximately 0.5 inches shorter than a Creedmoor or .243 Winchester. This puts some interesting demands on the magazine systems and the ejectors of bolt-action rifles.

Several companies in these markets have specialized in making adapter kits for magazines and rifle actions with inertial ejectors (aka, mechanical ejectors like a Mauser) so that the bolt-action receivers made for things like .308 will work well for 6mm BR, BR-Ackley, BRX, Dasher, etc. The speed that these cases bring to the table seems like it might be a detriment when compared to the bigger cases like the .243 and 6mm Creedmoor, but what they lack in speed they handily make up in single-digit SD variation, inherent accuracy, long barrel life and exceptionally low recoil. These benefits are what really draw the top competitors to such cases. 

6 BR-A & 6 Dasher

The repeatability at long range with high-BC match bullets is exceptional, especially with the latest developments in monolithic solids and computerized reloading scales. Shooters are regularly seeing reload performance with both ES and SD in single digits; many times with SDs are below 4 fps on 10-shot strings. The 6 BR is a long-neck cartridge with 30-degree shoulder and is known for legendary accuracy even at 1,000 yards. 

The BR-Ackley is a simple Ackley Improved variant with a little less body taper and a 40-degree shoulder, but it retains the same neck length as the 6 BR. Performance is about 50 fps faster than the regular 6 BR on average with all bullet weights. The Dasher is a full-on AI variant with a shortened neck, reduced body taper, and 40-degree shoulder. It also has a lengthened powder column that is typically 100 fps faster than the parent BR case and demonstrates very good barrel life for a 6mm competition cartridge. It’s capable of meeting that 2,950- to 3,000-fps threshold with 105-grain bullets that many competitors seek. Barrel life is typically around 2,500 rounds.

Considering the relatively small performance difference between the 6 BR-A and the 6 Dasher, we will limit the tabular performance comparisons to the 6BR and Dasher only for brevity. Brass is readily available for the 6mm BR from several companies. Some are even making turn-key 6 Dasher brass now to allow shooters to skip fire forming.

6mmx47 Lapua

Now, let’s look at the middle ground made up by the 6mmx47 Lapua. It is about 0.25 inches shorter than the .243 and 0.25 inches longer than the 6mm BR case. It has about the same performance average, as well. Where the 6mm BR is a 2,850-fps performer for 3,000 to 3,500 rounds using 105-grain bullets, the 6mm Creedmoor-sized cases are around 3,100 to 3,150 fps for 1,500 rounds and the Dasher is 3,000 fps for 2,200 to 2,500 rounds.

We have a middle child here that is an easily formed wildcat off the 6.5×47 Lapua parent case. It’s very accurate, nearly as much so as the 6mm BR and variants. It has a 30-degree shoulder and long neck like the BR does. Lastly, it gains the benefit of feeding and ejecting without any special considerations for the magazine and receiver parts.

6mm ARC

The latest 6mm variation to cause a stir in the tactical/practical shooting competition world (PRS-style matches) is the 6mm ARC (Advanced Rifle Cartridge) from Hornady. The cartridge is a diminutive variant based on the 6.5 Grendel case, so is pretty much a SAAMI-approved 6mm Grendel. The original design was for precision semi-auto platforms such as the AR-15. However, that has not stopped bolt-action shooters from immediately looking to experiment with it.

The cartridge holds nearly as much powder as the 6mm BR. Also, the performance is very similar with factory Hornady ammo shooting 108-grain ELD-M bullets in the 2,800-fps range. It has a distinct benefit, especially for the casual shooter, varminter and non-reloader—good factory ammo.

I’ve been experimenting with the new caliber in both factory ammo and handloads. I’ve seen impressive results from such a small case. It does suffer similar magazine issues as the 6mm BR in some rifle actions. However, as shooters have come to accept the modifications necessary to use 6 BR variants, my strong suspicion is that the 6mm ARC will become well accepted soon enough in the bolt-action community. It’s also a perfect little case for the next ultra-hot .22 centerfire wildcat for long-range predator and varmint hunting.

Performance & Wind Calls

The accompanying table of performance is based on my personal rifle barrels in the various calibers. These are generally an average over several barrels. The lone exception is the .243 Winchester, of which I only own one example. The 6mm ARC is too new to have an accurate idea of barrel life, but it will be at least as good as the 6mm BR. 

6mm ammo, 6mm cartridge Performance Chart
The table has comparison values for wind deflection compared to a 6.5mm Creedmoor shooting a 140-class bullet at 2,800 fps to give the reader an idea as to why shooters using “little bullets” are seeing so much success in competition circles with the pea shooters.

To be quite honest, there is a huge disparity in the amount of powder used to launch the same bullet from 2,800 to 3,250 fps. You must practically double the amount of powder to go another 450 fps, which yields a notable but not impressive difference in the wind call at 1,000 yards. At 600 yards, there is very little difference.

The last couple of rows in the performance table really tell the story when we compare the total wind and the differences in wind call as compared to a hot 6.5 Creedmoor load. From the diminutive 6mm ARC up through the .243 Winchester, we’re only talking about 18 inches of wind-call difference.

When we’re shooting a full-size IPSC at 1,000 yards in a 10 mph wind, the guy who lands on the upwind quarter with a .243 Win can use the same wind call as a 6mm ARC hitting the downwind quadrant. It’s pretty impressive how close the wind performance really is for these cartridges.

Digging Into the Numbers

Unsurprisingly, there is an inverse correlation to speed and barrel life. Notable as well is that over the approximately 350 fps performance spread, there is a trend toward the slower, more accurate, lower-recoiling cases with longer barrel life. Are we racing here?

Yes and no. We’re racing, but we’re racing toward repeatability and predictability. Utmost performance comes at the cost of high maintenance costs. People always work on Formula 1 cars, as they do rifles like a .243 Ackley shooting 115-grain match bullets at 3,400 fps. Load tuning, barrel replacements, brass maintenance, carbon rings … these are things that shooters accept up to a point. They want the performance, but there is a limit to the hassles they will go through to get it; that’s especially the case when we consider the wind deflection performance as seen in the comparison table shown above.

Wind-drift comparison across the various cartridges as compared to the 6.5 Creedmoor. What little these cartridges give up in wind performance they make up for in incredible downrange consistency.

In competition circles we don’t really worry what the drop is. Not in a comparative sense anyway. We worry about the wind. Specifically, what we’re worrying about is the variability in the wind and the shooter’s capability to correct for that wind. By using a cartridge with better ES/SD values and bullets with better BC values, as well as more consistent BC values, the performance of the rifle becomes more predictable. More predictability of the rifle means less variables that a shooter must consider when taking a shot.

What Does it All Mean?

So when we take a look at the 105-class bullet wind deflection at 1,000-yard targets and we boil it down to an error window from shot to shot, the difference of 50 to 100 fps on a given cartridge comparison is relatively small. The edge that a little more velocity brings is often negated by the extra maintenance incurred on hotter chamberings as well as the additional recoil in hastily supported (barricades, tires, rock piles) or self-supported (sling shooting) situations. Knowing where the shots went and seeing the trace off the bullet as it flies is invaluable data that reduced recoil gives us. 

I started with a .30-06, .243 Winchester and .260 Remington about 20 years ago. Then I moved from a 6.5 Creedmoor to a 6×47 over several years and a couple dozen barrels. Then I went to a 6mm Dasher and built a 6BR trainer only to realize how convenient the 6 BR is without fire forming. Now, with less time to reload than ever before and some generous friends in the ammunition world, most of the 6mm I shoot is 6mm Creedmoor and some 6 BR; though the advent of the 6mm ARC has drawn my attention as well.

The Upshot

All of the previously discussed calibers are capable of taking whitetail-sized game with proper shot placement; my friends and I have done that from near point blank (24 yards) out to distances approaching a half mile. The calibers can do it. And if the shooter is up to task there is nothing that says a 6 ARC or 6BR isn’t an effective whitetail chambering anywhere inside of several hundred yards.

The 6mm BR holds numerous 600- and 1,000-yard world records in the benchrest world. It is also a perennial favorite amongst F-class shooters for midrange competition. Meanwhile, the bigger 6s and 6.5s have largely dropped out of favor for heavy 7s and heavy 30s for 1,000-yard open competitions.

But in PRS, everything is going to smaller, faster, lighter calibers in heavier and heavier rifles that have muzzle brakes and suppressors. These configurations are put together so that the rifle shoots well under free recoil and the competitors can see trace and impacts during improvised and unsupported stages, all while giving up as little as possible in the exterior ballistics.

Final Thoughts on the 6mm

By now, readers might be asking: “What’s the best one? Which one should I use?” There’s not a truly one-size-fits-all answer here. This is a high-level overview of six chamberings that envelope probably 30 more that fall inside that spectrum.

Shooters who don’t [always] reload should look hard at the 6mm Creedmoor and 6mm ARC. As mentioned above, there are a lot of great factory options for them now.

Folks that are reloaders but don’t have experience forming and fire forming might want to stay clear of the Dasher. However, they would be right at home with the 6×47 and 6BR or comfortable enough with the 6mm BR-Ackley.

And for folks who are reloaders but have budget concerns about the cost of barrel expenses and powder, then the 6mm BR and 6mm ARC give them the best bang for their buck, as long as their chosen receiver is capable of ejecting reliably. Finally, if the receiver isn’t up to task, then there is the superbly accurate, easy-to-load 6×47 Lapua.

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