Cartridges - 6XC For Long Range Prone

A Prone Shooter Looks at the 6XC
by Germán A. Salazar
This article was originally published in the May, 2007 issue of Precision Shooting magazine.  The article also appears here: http://www.6mmbr.com/6xclongrange.html

What is the 6XC and why should we care about it? The 6XC is one of the latest in a long series of modified 22-250 cases necked up to 6mm; a legacy that probably goes back as far as the 22-250 itself and perhaps peaked in popularity with the Donaldson and Walker versions of the 6mm International. While those early efforts have faded from the scene, newer versions continue to crop up and chief among them today is the 6XC. Figure 1 shows the 6XC between the 6BR and the 6.5-284 for comparison purposes.

Most US Highpower competitors are at least aware of the existence of the 6XC cartridge; our overseas readers perhaps less so. As Vince Bottomley indicated in his article in the July 2006 issue of Precision Shooting, the 6XC is very similar to the RUAG 6x47 Swiss Match. The 6XC has been used to win the NRA Highpower Rifle National Championships at Camp Perry for the last several years by its originator David Tubb as well as by Norm Houle and Dennis DeMille. Mr. Tubb also won the 2005 NRA Long-Range Championship firing the 6XC. That achievement made an impression on many dedicated prone shooters who had previously given the cartridge little notice. Today, the 6XC is making inroads at prone matches from 300 meters to 1000 yards. This article is a look at the 6XC from the perspective of a dedicated and very active prone shooter; it is a compilation of my experience, observations and opinions of the cartridge’s advantages and disadvantages for prone shooting only. As with all opinions, yours may well differ from mine and there’s certainly nothing wrong with that.

While the 6XC has been closely associated with McMillan’s Tubb 2000 rifle in which it is offered as a standard chambering, it is a simple enough matter to chamber any existing rifle with a 0.473" bolt face for the cartridge. Now that C.I.P. standardization is in place and Norma is prodcing brass, I expect other makers to follow. My test rifle for this article is a Winchester Model 70, stocked by Alex Sitman of Master Class Stocks about ten years ago and recently barreled by Clark Fay of Raton, New Mexico with a Krieger 6mm, 1:7.5" twist barrel in the now ubiquitous 30" medium Palma contour in place of its former 30-06 Krieger.


Chamber Reamers
6XC chambering reamers are available directly from Pacific Tool & Gauge (PTG) as well as from Hugh Henriksen. I ordered a reamer from PTG with a 1°30" leade angle and a 0.271" neck diameter (see Figure 2). The standard Henriksen reamer is made with a 0°45"leade angle and a 0.276" neck diameter. While the shallower leade angle seems to be gaining some popularity in various Highpower oriented reamers, I have yet to see a cogent explanation of why it might be better. There are those who argue for and against the shallower leade angle each claiming that it will give longer or shorter throat life, more or less accuracy. Until I see a valid direct comparison, I will remain tradition-bound at 1°30". I selected the 0.271" neck diameter based on the neck thickness of Norma 22-250 brass as I wanted a reasonably close fit without neck turning; this gave me 0.004" clearance. Subsequently, when the Norma 6XC brass became available I found this dimension to be too tight as the 6XC brass has a slightly thicker neck. Turning the 6XC case necks to 0.013" solved that problem.

Brass
The brass cartridge case is, of course, the heart and soul of any discussion of a new cartridge and in the case of the 6XC it has also been its Achilles heel. Whether forming cases from 22-250 and putting up with the same donut problems that have plagued reformed 22-250 cases since the time of Harvey Donaldson, or trying to buy Tubb 6XC brass as offered by Superior Shooting Systems (SSS), the life of the would-be 6XC shooter hasn’t been a bed of roses. Over the past three years, there have been three separate batches of this US made brass and all have different internal volume so proceed with caution on load data when using Tubb headstamp brass. To further complicate matters, delivery times on orders have at times exceeded two years aggravating potential customers to the brink of despair. More than one competitor I interviewed gave up on the 6XC over the brass issue.

In August, 2006, however, Norma began importing 6XC brass into the US and it is of their usual high quality. Delivery of the Norma brass remains a single source proposition for the time being; hopefully the supply issues that have dogged SSS in the past will be overcome and delivery times will come into alignment with the rest of the industry. Far better would be for Norma to distribute this brass through all of their usual distributors; we can only hope for such a decision to be made.

It is fair to say that the majority of brass being fired in 6XC chambers today is reformed 22-250 brass of various makes. Remington, Winchester and Norma 22-250 brass are readily available and all are suitable for reforming; we will examine all three (see Table 1).

Prior to the arrival of the Norma 6XC brass, my primary choice in brass was Norma 22-250 because of its great uniformity in case and neck thickness as well as weight and volume. Norma is somewhat more expensive on initial purchase than the others but that difference is minimal when amortized over the life of a case and worthwhile for the higher quality. I have fired one set of Norma cases twelve times with full power loads with no detectable loosening of primer pockets, neck splits or any other form of case failure or fatigue. The Norma 22-250 brass tends to form a donut in the lower portion of the neck after fireforming which can be reamed out with a K&M neck turning arbor with the carbide cutter. I have also detected the donut in Winchester brass, but not in Remington; however, I should mention that I have used far less of those brands and different lots may provide a different result.

Three fundamental changes are made to 22-250 brass in the process of becoming 6XC: the neck is expanded to 6mm, the shoulder is pushed back and the body taper is greatly reduced. Fortunately, 6XC case forming is as simple as any reforming operation can be; one simply runs a 22-250 case into the 6XC sizing die and out comes a case suitable for firing in competition, albeit appearing somewhat malformed. After forming but before firing, Winchester brass displays the characteristic mushroom shape at the shoulder, whereas Remington and Norma brass are not quite as dramatic looking (see figure 3).


For those who can’t abide the thought of mushroomed cases, Larry Medler has developed a simple “No-Mushroom” process which he details on his web page: (see figure 4) While Larry’s method is interesting and produces cases that present a more pleasing initial appearance, once the first firing takes place and the case blows out to its final dimensions, the initial shape is moot.

The new Norma 6XC headstamped brass is what everyone has been waiting for and the initial lots I received live up to the expectation. I got 200 cases from Swedish competitors at the 2006 ISSF World Championships in Zagreb through a friend who was there and another 300 cases through the US distributor. Both lots are of very uniform construction with suitably uniform necks (less than 0.001" variance). Internal volume is slightly greater than the reformed Norma 22-250 brass and weight is proportionately lower (See Table 1). After four full power 1000 yard loadings, the 6XC headstamped brass shows no signs of expanding primer pockets. I intend to keep firing the same 70 cases until they show some signs of fatigue but I can’t predict if or when that will occur. One real benefit of the 6XC brass is the lack of donuts forming in the necks as compared to the reformed 22-250 brass. This is especially useful when firing 115 grain bullets as these tend to have their shank far down the neck into the donut area.


Internal capacity of Remington 22-250, Winchester 22-250, Norma 22-250, Norma 6XC and Tubb (3rd Generation) brass (all after fire-forming) is very close, allowing safe interchange of load data between these brass types. However, first Generation Tubb brass is very heavy (approx. 172 grains) and has much less case capacity – it is not suitable for the loads discussed in this article. I have not been able to locate any Tubb 2nd Generation brass for measurement.


Norma’s new 6XC brass uses a large rifle primer and a standard 0.080" flash hole as does all the 22-250 brass and the Tubb headstamped brass (see figure 5). Many of us who were waiting for the arrival of the 6XC brass hoped it would use a small rifle primer and a 0.065" flash hole like the 6BR and 6PPC; alas, that was not to be. Interestingly, Lapua chose the small primer path with their new 6.5x47 brass which is very similar (nearly identical, actually) in capacity to the 6XC. While I might wish for the small primer case in 6XC, the reality is that for 600 to 1000 yard shooting, the performance of the 6XC has been so good that it would be churlish of me to do so. At 300 meters on the ISSF target, however, every little bit helps and a I believe a small primer would be worthwhile.

Components
When developing loads for a new (to me) cartridge, I prefer to standardize on a few components to keep variables to a useful minimum. Hodgdon’s H4350 and H4831SC are the most widely used powders in the 6XC and were the primary powders evaluated here. Both of these powders’ burn rate sand bulk densities are well matched to the volume of the 6XC case with bullets in the 105 to 115 grain range.

Along the same lines, the Russian primers sold under the PMC label, have proven themselves to me in various calibers to have more uniformity in ballistics and more resistance to blanking than other types and they have become my standard primer. CCI-BR2 primers are a useful alternative.

Bullets are the big choice when it comes to most cartridges and this one is no exception (see Table 2). I normally shoot Berger bullets and their 105 VLD has proven to be an excellent choice for the 6XC. As I have always noted with Bergers, bullet weight and bearing surface length are very consistent. I do not routinely check these items on Bergers because their quality control is more than adequate; something that can’t be said for all large bullet makers. In my rifle, I can drive the 105 VLD at just over 3000 fps with accuracy almost equal to the 6BR at 2750 fps (my pet load). This achieves my initial goal with the 6XC which is to drive the same bullet I use in the 6BR to a significantly faster muzzle velocity without getting into dangerous pressure levels and without a big reduction in accuracy. Reduced barrel life compared to the 6BR will be the main trade-off, but that’s a fair price to pay for the added velocity.

Hornady’s 105 grain AMAX bullet is another good choice but I have only done a limited work-up on it at this point. The most intriguing bullets for the 6XC, however, are the Berger 115 VLD and the DTAC 115 grain bullet manufactured by Sierra. One of my goals in the preparation of this article is to give a useful comparison of the 105 and 115 VLD Bergers and the 115 DTAC bullets at 1000 yards with loads tailored to each.

Other bullets shown in Table 2 were evaluated and shot in the 6XC to some extent, but none equaled the accuracy and low wind drift of the Berger 105 and 115 in my testing and thus the focus of the test was kept on those two bullets. Readers should take a close look at the bearing surface length for each bullet as this has a measurable effect on pressures with a given load. A load that is maximum with a short bearing surface bullet may be in dangerous territory if a longer bearing surface bullet is substituted without a reduction in charge weight.

Dies
If there is one area of the 6XC project where I was initially frustrated, it was the loading dies. SSS offers a full-length sizing die packaged with a Redding Competition seater die. The seater is a typical Redding Competition Series die and works perfectly, leaving nothing to be desired in the way of improvement. The same cannot be said for the full length sizing die in the package. To be clear about the source of my frustration, I do not shoot any course of fire requiring rapid fire; my concern is purely with deliberate, precision shooting from the prone position at various known distances from 300 meters to 1000 yards. Accordingly, I prefer a full length bushing die that sizes the case minimally, a good selection of bushings and I also like to have a neck sizing die. None of these is possible with the SSS die set which uses a non-standard neck/shoulder bushing (available in only two sizes) and sizes the base of the case very aggressively (see figure 6). If one is an advanced reloader, a specialized die with multiple interchangeable non-standard parts for sizing (such as the highly regarded multi-caliber FL die from Warner Tool Company) can be acceptable. However, as this is the only generally available full length die in 6XC, it would have been much better for SSS to stay with the standard 0.500" diameter Wilson/Redding type bushings.

When the 2006 Redding catalog arrived I noticed that they planned to offer 6XC dies in various configurations. Since Redding dies are my usual choice, I was very excited at the prospect of getting dies that use my existing supply of bushings and that, perhaps, would be dimensioned more appropriately for my purposes. Redding now offers their Type S Neck Sizing bushing die, a body die and their Competition Seater as a set or individually as well as in the set (see figure 7).


I ordered the new dies and after a delay of some months while production cranked up, the Redding Type S bushing neck die and body die arrived. What a pleasant experience it was to have dies that work in the usual manner, don’t distort the brass and accept my existing supply of bushings. While a Type S full length die would be nice to have as well, I won’t quibble with Redding’s decision not to make that at this time as they have solved my problems with what they are making.

Unlike the Redding dies, the SSS non-standard neck bushing incorporates the 30° shoulder; obviously, this does not interchange with the existing inventory of Redding and Wilson bushings that suppliers and reloaders have in their inventory. While the SSS bushing sizes the neck all the way to the shoulder rather than leaving about 0.030" unsized as do the Redding/Wilson type, to me this provides no useful gain while significantly reducing tuning flexibility since only two bushing sizes are offered (0.268" and 0.266").

Even worse than the lack of a useful bushing selection in the SSS die, is the fact that the die sizes the base area of the fired case such an extreme amount that case life will, in my opinion, be adversely affected. As supplied, the sizing die reduces the base of the case by 0.0025" to 0.0030"; in fact it even appears to be reducing the solid web of the case. Fired cases measure 0.4680" to 0.4685" just above the web; when sized with the SSS die they measure 0.4655", far more than needed to ensure reliable function for my purposes. I had the die modified by a tool and die maker and it now sizes to 0.4665" for a 0.0015" reduction and also has a smoother feel which I believe is due to a better interior finish after the polishing. The Redding body die sizes the base of the case only to 0.4670" to 0.4675", a far more useful dimension than the SSS die and one that should lead to longer brass life.

Prior to the arrival of the Redding body and neck dies, a solution to the over sizing problem was neck sizing with a Redding 6BR bushing neck die. While the interior of the die is not a perfect fit on the 6XC brass (the 6BR has a slightly larger shoulder diameter) it works well enough. If you already have a 6BR die and don’t want to reset the lock ring, buy two Redding die spacer kits part number 80901 and use both 0.125" spacers and one 0.135" spacer under the die. You can now neck size 6XC with your 6BR neck die at the same lock ring setting. My resizing procedure is to full length size with the Redding body die or the modified SSS die for a 0.001" shoulder setback using the supplied 0.266" bushing, followed by neck sizing with the Redding 6XC neck die and a 0.264" bushing. This is for unturned Norma 22-250 brass which has a loaded neck diameter of 0.267". With the Norma 6XC headstamped brass, different size bushings will be required, the exact one depending on the extent to which the necks are turned (if at all). In my case, with necks turned to 0.013" and a loaded diameter of 0.269", I use a 0.268" bushing in the FL die and follow up with a 0.267" bushing in the neck die.


Loading and Shooting
My initial load workup centered on H4350 and the Berger 105 VLD. Working up in 0.2 grain increments I went from 37.0 to 40.0 grains. Accuracy, as measured by 500 yard groups fired prone, increased with each increase in charge weight up to 39.0 grains. I stopped testing at 40.0 because the primers were just beginning to show some slight cratering, although there were no other signs of excessive pressure. Bolt lift and extraction remained the same as at the lighter load levels, primer edges were still well radiused and the base diameter of the case did not show excessive expansion. Nonetheless, at nearly 3100 fps, and with cratering starting, this is a prudent place to stop.

A similar process with the 115 grain bullets led me to a maximum load of 43.0 grains of H4831SC with both brands of 115 grain bullets. Other competitors have reported good results with this combination and it fills the case more completely. Testing at 1000 yards showed the Berger 115 to have about 2 moa less drop and more resistance to changes in wind speed. At the recent 2007 Arizona 1000 Yard State Championship and a club match the preceding week, I fired six scores of 199 and 200, all over 10X’s with the 115 Bergers and H4831SC, all but one of these scores was with iron sights. That performance was good enough to make me rethink the usefulness of the 6.5-284 which shoots no better and recoils substantially more. The recoil is a factor for me as I shoot the entire string with the rifle in my shoulder and low recoil helps maintain consistency when shooting in this manner.

The missing element at this point is testing the 105 grain bullets with H4831SC at 600 yards and below; but others have reported good results with this combination. Conversely, when I tested H4350 with the 105 and 115 grain bullets at 1000, results were not as good as with H4831SC. It appears that for the prone competitor looking for one powder for the 6XC, H4831SC may well be the best choice.


Borescope examination of the 6XC bore at 1000 rounds revealed no significant cracking of the bore surface and minimal erosion. This compares very favorably with the 6.5-284 which typically shows very advanced cracking and erosion by this stage. For comparison, I examined my current 6.5-284 barrel which had 480 rounds at the same time and it was definitely in worse shape than the 6XC with twice the rounds fired. Given the performance of the 6XC at 1000 yards, I believe the comparison to the 6.5-284 is more relevant than to the smaller cartridges such as the 6BR. Obviously, the 6XC will have a shorter barrel life than the 6BR, but at this point I can’t determine how much shorter. I typically see 3500 to 4000 rounds fired on a 6BR before accuracy at 600 yards becomes suspect, I would imagine the 6XC might go 2500 rounds or so based on what I’ve seen so far, but that is nothing but raw conjecture.


Conclusion
After shooting the 6XC for one year at various distances and under wind conditions ranging from mild to severe; I can say that it is a great choice for 600 and 1000 yard matches. In fact, if I could only have one cartridge for 300 Meters to 1000 yards, this would likely be the choice; thankfully that’s not the case. In ISSF type 300 meter prone matches as well as 500 yard prone NRA matches and most 600 yard matches, I believe the 6BR with the 105 Berger VLD is still a better choice for pure accuracy. The 6XC’s greatest virtue, reduced wind drift with 105 and especially 115 grain bullets, is less of a factor at these distances than on a blustery 600 yard range and at 1000 yards, and at those longer distances, it is a winner and worth the effort.

SCROLL DOWN FOR TABLES







Brass Type


Water Capacity (Grains)


Avg. Case Weight (Grains)


Neck Thickness


Loaded NeckDia.


Norma 6XC


49.3


157.7

0.0135"
± 0.0004"


0.270"


Norma 22-250

47.8

161.2

0.0121"

± 0.0002"

0.267"

Remington 22-250

48.4

154.9

0.0135"

± 0.0010"

0.270"

Tubb 6XC (3rd Gen)

48.6

151.1

0.014"

± 0.0005"

0.271"

Winchester 22-250

48.7

161.7

0.0135"

± 0.0005"

0.270"








Bullet

Diameter

OAL

Bearing Surface

105 Berger VLD

0.2432"

1.237"

0.368"

105 Hdy. AMAX

0.2434"

1.230"

0.450"

105 Lapua Scenar

0.2430"

1.252"

0.450"

107 Sierra MK

0.2432"

1.227"

0.415"

108 Euber

0.2428"

1.218"

0.490"

115 Berger VLD

0.2431"

1.333"

0.414"

115 DTAC

0.2436"

1.293"

0.480"
 

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