Shooting the 6mm Ackley Improved

Back in 1965 I had the late, great Bill Marden build me a rifle using a new commercial FN Mauser action, fitted with a Sako barrel some 600mm long, chambered in 6mm Ackley Improved. He fitted a Fajen thumbhole stock which was one of the first of its kind ever seen on a sporter in those days. Over the years, I used that rifle very successfully on everything – from rabbits and crows to feral goats and pigs as well as fallow and red deer and a couple of sambar.

4000 shots, resultant throat erosion, 200 foot-seconds velocity drop and reduced accuracy ensued. Incapable of hitting a rabbit, in 1990 I had 600mm Sprinter barrel fitted. Ten years later I jettisoned the stock, had the rifle rebarreled to .240 Weatherby and fitted with a Tyack classic-style French walnut stock. I still have that rifle.

After gaining experience with the .240 Weatherby, I had no hankering to own another 6mm. At least I didn’t until in July 2012 I was offered a Mauser rifle with worn out barrel, a 3-die set in 6mm Ackley Improved and a new Hogue stock to suit. The rifle came with several hundred fireformed cases and reloads which gave me a serious attack of nostalgia. Despite attempts to wean myself off wildcats and improved cartridges, you can guess what happened?

After I looked up my article entitled “The .240 Hellcat,” which appeared in the June 1966 issue of Sporting shooter, I was convinced that the 6mm Remington and I had some unfinished business to attend to. To tell the truth I was embarrassed when I read the list of estimated velocities for my loads which credited the 6mm Ackley with bullet speeds that would strain the .240 Weatherby. But this was in the days before cheap chronographs became available and the velocities were probably taken from Ackley’s Handbook which contains a lot of over- optimistic data, including Mashburn’s figures for his .244 Improved which listed some unrealistic figures – up to 3450fps with a 100gn bullet!

Speer’s Reloading Manual for Wildcat Cartridges Number 4 which came out in 1960 listed chronographed loads for the .244 Improved which I was able to duplicate years later when I obtained a Telepacific chronograph. It drove a 75gn bullet at 3550fps; a 90gn bullet at 3300fps; a 100gn bullet at 3160fps; and a 105gn Speer close to 3100fps.

To kick things off, a new medium-weight Pac-Nor chrome-moly barrel blank was ordered from Phoenix Sports and I turned all the various components over to my friendly neighbourhood gunsmith, Rob Spittles, in Mudgee.

The Brno Mauser 98 action came with a Blackburn trigger, Jantz two-position wing safety and had bottom metal from an Argentine Mauser. Rob did an excellent job of fitting and chambering the new barrel to take the fireformed cases that came with the gun, attached a 2-piece Leupold mount and bedded the barreled-action into the Hogue stock leaving the barrel free- floating. After I mounted a Meopta Meopro 4-12x50mm scope, the outfit weighed 4.5kgs field ready. That amount of heft is enough to place it firmly the varminter category, but what I was really looking in the second incarnation was a genuine dual-purpose rifle.

Taking its place alongside my hot .220 Wilson Arrow, long- ranging .240 Weatherby and .25 WSM, the 6mm Ackley adequately fulfills the twin roles of a simonpure varmint rifle and a medium-calibre big game rifle. It can be suited to any specific task simply by handloading the bullet best suited for each individual hunting situation. When the original .244 Remington was introduced in 1955, the company touted the cartridge as being a “super” varmint cartridge rather than a deer slayer. Naturally, they gave it a 1:12 twist for long range accuracy with 75 and 80gn bullets, and loaded factory ammo with 75 and 90gn bullets. On the other hand, the .243 Winchester announced the same year, was sold as a dual-purpose cartridge and rifles had a 1:10 twist which handled bullet weights of 100 and 105 grains for deer. These 6mm longsters wouldn’t stabilize in the 1:12 twist of the .244.Naturally, the .243 outsold the .244 by a wide margin and Remington had to eat crow.

In 1963, the .244 was renamed 6mm Remington and given a faster 1:9 twist. Despite being a fine cartridge, one which offered an edge of 150 fps over the .243, the 6mm Remington never caught on and has since become moribund in the marketplace. Today, Remington no longer chambers any rifles for the 6mm Remington, but the .243 is chambered in 26 different variations in “Big Green’s” wide range of rifles. Remington ammunition offers one solitary 6mm load with a 100gn Core-Lokt PSP bullet at a nominal 3100fps. A sad fate for a cartridge that many handloaders consider superior in design to the .243.

A lot of the preliminary load development work had already been done by the previous owner of the rifle and his reloads were well within safe pressure limits. I used them to sight in the outfit and made them a starting point for working up new loads. It was 1976 before I got a Telpacific chronograph and started measuring velocities. At that time I was using what powders were available – H4831, Norma 205, WIN-760, IMR 4320, IMR 4350 and IMR 4831. In the second barrel (Sprinter), I tried AR2209 and AR2213, WMR, WXR and AR2213SC. This time around I added Re-22, Supreme 780 and BM8208.

Back in July 1966 I wrote an article about improved cartridges called “Blowing Up The Hotrods,” and the heading read: “It’s wise to be lukewarm about some of the so-called hotrod cartridges which are more bang than ballistics and more hot air than cold fact in performance. Sometimes the only thing not flat is the trajectory.”

I am not a rabid fan of improved cartridges, preferring to choose a larger capacity case rather than be bothered with fireforming brass, however, experience has taught me that they do have certain advantages. A good many older cartridges can be improved in design to produce more efficient burning of modern powders, but any increase in velocity is usually quite modest – usually 100-150 foot-seconds. The reason given for improving a cartridge is to get higher velocity with safe pressures. The most efficient load in any cartridge is the one that gives the highest velocity – lowest pressure ratio. And to do this in the 6mm Ackley you have to use the slowest burning powder that will give the required increase.

The .244 Remington originally appeared as a wildcat based on a necked-down .257 Roberts case with a 26 degree shoulder. It was developed by Fred Huntington of RCBS and given the title of .243 Rockchucker. It was Arnold Juenke of ICIL, I believe, who first produced an improved version of the .244. But it was further improved by Warren Page whose “Super Pooper” had minimum body taper and a 28 degree shoulder. Mashburn also had a finger in the pie and he claimed his version was the most powerful 6mm he had tested that could be fireformed from a factory cartridge. Later Ackley produced a more radical version with extremely sharp 40-degree shoulder, which is the design my rifle is chambered for.

The case holds about 10-percent (5 grains) more powder than the standard 6mm Remington, thus only a modest increase in velocity is possible and requires a considerable increase in the amount of powder burned. In fact, it is going to take quite a bit more powder to even equal, the velocity of the standard case.

After carrying out considerable experimentation with improved cartridges, my conclusion is that while combustion is improved, there is little increase in velocity without reaching higher pressures. But there is plenty of evidence that cartridges like the 6mm Ackley will withstand higher pressures than the standard case.

The generally accepted explanation for this is that the straighter sides of the improved case grasp the chamber walls to an extent which reduces backthrust against the bolt to a measurable degree. This apparently does away with the tendency of the bolt to spring back from the thrust of the cartridge as it is fired, like it does with more tapered cases.

It has been successfully demonstrated that straight-sided cases with minimum body taper have been loaded heavy enough to blow out the primer, badly expanding the case head in the process and yet still extract normally. There seems to be ample proof to support this conclusion. Indeed such well known experimenters and ballisticians including P.O Ackley, Arnold Juenke, and Robert Hutton all supported this theory. However, some physicists beg to differ, insisting that all gases expand equally in all directions regardless of case shape.

The things we do know for sure is that improved cartridges fired at higher than normal pressures extract easier, show less head expansion, less enlargement of the primer pocket and less lengthening even after being reloaded many times. On the other hand standard cartridges show all of these things to varying degrees. Both the .22-250 and .220 Swift stretch enough to require frequent trimming and some excessively tapered cases like the old .300 H&H and .375 H&H are prime offenders in all respects.

Another advantage of a sharp-shouldered case is that it creates greater resistance to the burning powder, holding more of it back in the case during the burning process, thus preventing unburned powder granules from being blown out into the throat of the chamber to give a sand-blasting effect. This lessens throat erosion which is the main cause of loss of accuracy in a great many barrels, especially those using heavy charges of slow burning powders.

My opinion of the 6mm ackley Improved (or any other improved cartridge) is that while it gains better combustion and longer case life, accuracy doesn’t enter into the equation since it is affected more by the quality of the barrel, chamber and reloads. For the true believers however, the slight gain in muzzle velocity may make the conversion seem worthwhile.

My 6mm Ackley underwent load development with bullets weighing from 62 to 100 grains, since I don’t have much use for ultralight 55 grainers which have poor ballistic properties. Subsequent results showed that the 6mm Ackley was comfortable with light 62 and 65gn bullets used with faster powders like BM8208 and AR2208. Both the .243 Win. and 6mm Rem. are capable of driving the 55gn Ballistic Silvertip at around 4100fps and the .240 Weatherby doesn’t seem to do a lot better. But the short, fat .243 WSSM (identical in capacity to the .243 Ackley) manages to squeeze out another 100 fps. The Weatherby comes closest to the .243 WSSM, but burns nearly 10-percent more powder to do so.

The serious long range varmint hunter would probably pass over the less-then-efficient 55gn bullet with its low SD of .133 and BC of .276 in favour of the Hornady 65gn V-Max (SD .157 and BC .280) which will retain more of its initial high velocity over the long haul and lies closer to the wind.

Bullet choices for testing included 62, 65, 75, 80 and 85- grain weights for varmint-predator work and 90 and 100-grain weights for medium to big game. Makes included Hornady, Sierra, Nosler, Barnes, Winchester, Speer and Berger. Loads in the table all proved safe in my rifle and most of the powders gave a high load density, but even the slowest burning numbers are only lightly compressed.

Testing the rifle with varmint bullets resulted in some rather impressive velocities. For example, the 62gn Varmint Grenade clocked 3905fps with 45gn of BM8208. But the most accurate load was the Hornady 65gn V-Max over 49gn of AR2209 for almost 3700fps. Equally as accurate and a real eye opener was the Hornady 75gn V-Max and 50gn of AR2209 for 3710fps. For comparison, the Hornady handbook lists a top speed of 3400fps with that bullet in the 6mm Remington.

Both these loads averaged a half-MoA for five 3-shot groups at 100 yards. Both loads are a couple of grains below maximum; although I got 3897fps when I increased the charge to 52gn of AR2209 behind the 75-grainer, it nearly doubled the size of the groups. This was the only load that produced a slight resistance to bolt lift, extractor marks on the cases and flatter, cratered primers.

The 85gn Nosler Ballistic Tip and 50gn of AR2213SC produced 3310fps , but 51gn of Re-19 hit up 3465fps with groups hovering around .450. Similarly, 51gn of Supreme 780 drove the 90gn Berger at 3300fps giving nice round groups of .425. And the Hornady 65gn V-Max and 41gn of BM-2 churned up 3610fps and shot into .70. With this bullet I prefer to stick with 49gn of slower AR2209 and 3688fps and .500 inch.

For big game I like a stoutly structured 100-grainer in my .240 Weatherby which drives them at 3350fps; the 6mm Ackley got to treading close on the belted magnum’s heels by getting the 100gn Nosler Solid Base out at 3287fps ahead of 51gn of Supreme 780. I thought it a bit unusual but pressures seemed a bit milder using 44gn of WIN-760 to get the Sierra 100gn HPBT going 3260fps – hardly a worthwhile difference.

My 6mm Ackley was most catholic in its tastes. The majority of loads grouped under .75, with the best loads running below .50. During prolonged range sessions, swapping between my .220 Wilson Arrow and .270 WSM, the Pac Nor barrel hardly got more than warm. Its beefy profile undoubtedly helped keep it cool.

Summing up: I’d place the 6mm Ackley on level with the .257 Ackley Improved Roberts which receives a lot of favourable press. But the 6mm Ackley may be a more viable cartridge for the man who wants a true dual-purpose rifle. It has a sufficiently large boiler room to load the heaviest 6mm bullets and slower burning propellants for use on big game, while for small varmint and predators at extreme distances, the lighter bullets are devastatingly effective. There ain’t many improved cartridges I’d make room for in my gun safe, but I’ll make an exception of the 6mm Ackley.

This article was first published in Sporting Shooter Magazine April 2013




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