With the explosive growth in precision rifle, long range and racegun shooting sports the past few years, the constant search for a better or ultimate bullet for these purposes has also intensified, with guys necking down, wildcatting or blowing out the shoulders of every single cartridge they can find. The focus being especially on 6.5mm and 6mm bullets, as these seem to be the sweetspot for precision rifle and race gun shooting in terms of BC proportions, recoil and muzzle velocity. Because of the lower felt-recoil often offered by the lighter 6mm bullets, and the higher muzzle velocities they offer, they have become the most popular for race gun cartridges.
There is no denying that the .243 Winchester cartridge, introduced way back in 1955, has been the indisputed champion 6mm cartridge when it comes to popularity for any type of shooting, from hunting to competition gong shooting, so naturally the search for a better 6mm when including modern advancements in propellant and increased data and knowledge about ballistic performance, whatever is developed, will always be compared to the 243 Win. Because of the great Creedmoor boom of the 2010s, and the 6mm Creedmoor popularity following suit, that is the other 6mm that all newcomers are compared to. Then there is the new 6mm Super LR, or 6mm Super Long Range as it is fully named, with many just calling it the 6mm SLR or 6 SLR. Truly a hybrid between the 6mm Creedmoor and the .243 Winchester, is the better than both of them?
What is a 6mm SLR
First off, what is a 6mm? The 6mm, not-so-obvious to some and obvious to others, stands for the bore diameter of the barrel, more specifically actually a .243″ or 6mm bullet diameter. The bullet diameter is 6mm, the same as any other 6mm centerfire cartridge like the 6mm Creedmoor, 6mm BR, 6mm Dasher, 6×47 Lapua and the .243 Winchester. The SLR in 6mm SLR stands for Super LR, or Super Long Range in full, hinting to the fact the it is meant to be the ultimate in long range capabilities for a 6mm cartridge, with it also having been called “The Ultimate Long Range 6mm”.
The Creation
Essentially a 6XC with a .120″ longer case body. Around October 2006, Robert Whitley of 6mmAR.com, was experimenting with .243 Win brass and a 6XC sizing die screwed out a bit, in an effort to create a cartridge, that would be able to shoot the longer 115gr DTAC bullets closer to 3000fps with slower burning powders than was needed for the 6XC. He first called it the 6mm Super X. The 6mm Super LR was then ultimately refined from that initial 6mm Super X, when custom sizing dies were made up for the new cartridge design, which allowed the base diameter of the case to be slightly larger.
What Robert inadvertently achieved, is what many consider to be the ideal long range 6mm case for consistent accuracy, based on a few factors.
- With improved 30 degree shoulder angle, the 6mm SLR is less harsh on barrel throats, and giving you slightly longer barrel life than a .243Win, and because of the long neck, also longer barrel life than the 6mm Creedmoor.
- With the longer neck, ensuring more surface contact with the bearing surface of the bullet compared to a 6mm Creedmoor, ensure more consistent neck tension, especially where ares with micro-imperfections on the inside of the neck affects the neck tension. This ensure more consistent groupings, which was proven thanks to the notorious Houston Warehouse Project which proved neck tension to be one of the most important factors for consistent shooting and small groupings.
- With larger case capacity than the 6XC, it allows greater muzzle velocities to be reached before pressure signs, and can accommodate slower burning powders that are less temperature sensitive than what is needed in the 6XC.
Consistent neck tension, longer barrel life, ability to use variety of powders, allows use of BC class-leading 115gr DTac bullets, can be formed with easily available .243Win brass means that this may well and truly be the ultimate long range precision 6mm Cartridge for race gun and precision rifle competitions.
Brass
Initially to create brass for 6mm SLR cases, one would simply size .243 Winchester brass cases with a 6mm SLR full-length sizing die, load them with a mild to low load and then fire them through your 6mm SLR chambered rifle, this will push the shoulders out and shape the brass to the chamber, it is preferred to use softer brass like Peterson, Winchester or Hornady as harder brass like Lapua can cause doughnuts in the necks to form, which can be corrected through neck turning.
Shooters Decide
As mentioned previously in many of our other wildcat cartridge posts, with so many new cartridges coming out almost every month, it is hard to know what is worth getting and what not. Thankfully with just a bit of time, either through marketing-hype or shooter addoption because of true performance benefits, shooters quickly decide what gets adopted in the mainstream and what not, so it would be wise to wait out moving to any newly-developed cartridges until they have been around for a few years, and there is plenty of Dies and load data available for them.
Create Your Own
There aren’t currently any large scale manufacturers making a 6mm SLR Factory Rifle, however, they are very easy to create by just sending your .243win chambered rifle with a fast twist barrel, or 6mm Creedmoor rifle in to a competent gunsmith, and have them just redo the chamber of your barrel with a 6mm SLR reamer.
Creating a 6mm SLR chambered rifle is also fairly simple, you simply use the action of a rifle, or a rifle with a compatible bolt face and action length as can be seen on our bolt face database, and then have a gunsmith chamber and fit a .243 / 6mm barrel to it. The gunsmith would need to preferably have a 6mm SLRr reamer to create a chamber in the barrel for it. We used a Savage Model 12 LRP Stainless Action from a rifle originally chambered in 243 Win, fitted a .243 Lothar Walther barrel on it with the chamber cut for a 6mm SLR, and now we have a superb Savage 6mm Super LR rifle.
6mm SLR Dies
We have mentioned it a few times throughout this article so far, but really to reiterate, Lemi Shine works wonders for cleaning brass, removing the ugly green hue sometimes left after tumbling brass in a wet tumbler, and leaving the brass looking shinier than when it was new. You can also use Lemi Shine ultrasonic brass cleaners.
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6mm SLR Twist Rate
The longer and heavier 6mm bullets that this 6mm SLR cartridge is designed for and excels with like the Berger 109gr Hybrid Target (as pictured in the title image at the top)and the 115gr DTac, a fast twist of 1:7 is ideal, but 1:7.5 and 1:8 twist will also do just fine.
6mm SLR for Hunting
Depending on size of the game hunted, but generally you will often hear most industry authorities recommending 6.5mm, 270 Win or 7mm calibers for any smaller CXP2 type antelope, and 30-06 Spingfield or larger for CXP3 medium to large game. These general guidelines apply more to the older traditional calibers when speeds were much lower, bullet were shorter and much lighter, with modern cartridge advances, the velocities achieved by modern medium and smaller calibers make up for the energy lost in mass of the bullet, and the modern twist rates allows low drag heavier bullets to carry those velocities and energy further down range.
The 6mm SLR can comfortably take down most CXP3 size animals with well placed shots in the head or heart, if the bullets are expanding type bullets in 95 grain or larger size.
6mm SLR for Competition
Personally for most other field competition shooting for distances below 1500 yards, we recommend the 6mm Super Long Range above almost all other cartridges. It has wide forgiving sweetspots, which makes it inherently accurate even for the most novice reloaders thanks to it’s long necks providing even neck tension when loaded correctly, and the velocity combined with the ballistic coefficients of the .243 bullets in 105, 108, 109 and 115gr are so good, that wind does not bother it as much as even much larger calibers.
6mm SLR Barrel Life
Varying depending on your rifle, barrel, load, velocity, bullet barrel break-in and cleaning regime. There are many factors that determine the number of shots a barrel can take before the lands and rifling become worn to a point where the accuracy suffers. We have found some 6mm SLR shooters pushing bullet speeds in excess of 3300 fps to get around 1800 to 2200 shots out of their barrels, and some have been able to extend that with proper barrel care life letting the barrel cool between firing strings, and hBN coating their bullets. If you however stick with the more common mild load needed for racegun competitions to spot bullet trace, so in the area of around 2850fps, then you can easily expect 3000 or more shots from you barrel thanks to the sharp shoulder angle and long neck protecting the throat of the barrel from the blast.
6mm SLR Load Data
From our research so far, the powders mentioned that worked well with the 6mm Super LR cartridge was the Reloder 19 and Reloder 22 from Alliant, and the H4350 and H4831SC from Hodgdons.
Robert Whitley himself mentioned getting right around 3000fps with the 115gr D-Tac bullets, when loaded with 43.5gr H4831SC, but it would be wise to start much lower than that with your own rifle and work your load up in small increment checking for pressure signs.
6mm SLR vs 243 Win
The 6mm Super LR has a .321″ long neck, compared to the shorter .263″ neck of the parent .243 Winchester case, meaning more surface contact on the bearing surface of the longer bullets, ensuring more consistent neck tensions. The 6mm SLR has a 30 degree shoulder compared to the 20 degree shoulder of the .243 Win, which prevents the cases from stretching as much and lengthening when firing, and also protects the barrel throat better for slightly better barrel life.
6mm Creedmoor vs 6mm SLR
The 6mm Super LR holds 54gr of water weight capacity in it’s case, whereas the 6mm Creedmoor only holds 50gr of water weight. The longer .321″ neck length of the 6mm Super LR compared to the shorter .269″ neck length of the 6mm Creedmoor means the 6mm SLR has longer bearing surface contact and therefore more consistent neck tensions. This would mean that the 6mm Super LR beats the 6mm Creedmoor in just about all aspects other than ammo and reloading component availability as well as factory rifle chamberings.
Great Review of a great cartridge