260 Remington Ackley Improved Chambering

Switch Barrel for 6mm Dasher Action and Stock

Click Here For The NEWEST ENTRY

First Trials

I used an old 6.5mm barrel from the 6.5-284 TubeGun, set it back, and rechambered it in 260 Ackley Improved. I used a 6.5-243 Improved reamer from Dave Kiff at Pacific Tool and Gauge, with the throat length set for the Sierra 142 grain MatchKing bullets. Since the TubeGun design has no recoil lug, I am able to easily switch barrels in the field, giving me both 6.5mm caliber options. I will be using Remington brass for the initial tests.

Based on my QuickLoad studies, I expected that the 260AI cartridge would yield the same ballistics as the 6.5-284, with about three grains less H4350. The results were exactly as predicted. The cartridge achieved 6.5-284 velocities with significantly less powder than the 6.5-284. The best results were with 123 grain Lapua bullets (Moly) and 47.0 grains H4350. Initial tests with Ramshot Hunter were also promising. Remington 260 brass was used, and there were no neck splits out of 100 rounds fireformed. There were no donuts formed at the neck shoulder junction. The brass had 57.4 grains H2O capacity after fireforming.

Switch Barrel in 260AI for the 6mm Dasher

Based on the preliminary tests, I chambered a 29.5" Bartlein 5R, 8.5" twist, 1.25" straight cylinder contour for the 260AI, and fit it to the 6mm Dasher action and stock. Since the recoil lug is pinned, this makes changing the barrels in the field a breeze.

Here are the specifications for this rifle and barrel:

The specifications for the rifle are as follows:

Fireforming of .243 Lapua brass was done using 142 SMK moly bullets, over 47.4 grains Ramshot Hunter, and a GM215M primer. the .243 necks were expanded to 6.5mm using a K&M mandrel before seating the bullets. All cases fireformed well, with no split necks. I went with the Lapua brass for this experiment due to the lack of availability of 260 Remington brass from Remington, and the great intrinsic consistency of Lapua brass.  Accuracy was excellent, at less than 0.5" five shot groups at 100 yards during fireforming.

After fireforming, the Lapua cases had 55.9 grains H2O capacity. There was a pronounced donut at the neck shoulder junction, thick enough that a bullet would not pass after firing. Using a Starrett hole gage, this donut measured 0.005" smaller than the fired neck. A case was sectioned, and the donut is clearly visible, as well as the slight thickening of the neck walls at the neck shoulder junction:

The throat on the chamber is set so that only the 139 Lapua Scenar will not seat deep enough due to this donut. I ordered the reamer with a very short throat, and used a separate 6.5mm throating reamer to get the precise throat depth necessary to clear the donut on the longer bullets. The 142 SMK, 123 Lapua Scenar, and the 130 Berger VLD will all seat just back from the lands. A Forster inside neck reamer will be used to try and clean up these donuts. This may not be necessary, as it is anticipated that the 123 Scenars or the 130 Berger VLD will be the bullets of choice.

Load Development for the 260AI Barrel

1/27/07

Load development today used the fireformed Lapua .243 Win brass, GM215M primers, and Ramshot Hunter ball powder. Moly coated Lapua 123 grain Scenars, as well as moly coated 130 grain Berger VLDs were tested. Both bullets cleared the donut as 0.010" back from the lands.

Here is the full "safari mode" kit for load development at the Cascade Rifle and Pistol Club 200 yard "B" range in Ravensdale, WA. It takes me over 30 minutes to set all this up, if I have the range to myself:

Here is the rifle set up in the bags:

Note the snakeskin-like masked bead blast effect on the barrel. I used the protective plastic web-type barrel cover as a mask during bead blasting. It is hard to keep the cover from shifting as you manipulate the barrel in the blast cabinet. Sure looks good:

OBT loads at 1.346 mS were targeted for both bullets. The Lapua 123 Scenars ran 3120 FPS, with a SD of 10 and an ES of 25 at 48.8 grains Hunter. The 130 Berger VLD ran 3048 FPS, with a SD of 11 and an ES of 26 at 48.2 grains Hunter, and 3011 FPS, with a SD of 15 and an ES of 39 at 47.5 grains Hunter. QuickLoad confirmed that these were at the OBT.

There was a LOT of wind today, over 15 MPH, and switching all over, yet the groups were consistently at or under 0.5" at 100 yards. The best group was 0.234", with the Berger's grouping much more consistently (no flyers) than the Lapuas:

More testing at 200 and 600 yards needs to be done. The Berger bullets group much better than the Lapua. Not surprising, based on bearing length and Junke machine test results between the two manufacturers products. The Berger's were nearly perfect, and the Lapuas were so-so. The focus will be on the Berger's for the long range load development and proofing.

2/3/07

Tested the 123 grain Lapua Scenar and 130 grain Berger bullets on the Juenke concentricity machine. The results clearly showed the much better dimensional tolerances of the Berger's:

The Berger's had over 93% of the 80 bullets in the sample showing less than 10 deviations units, and less than 3% over 15 units. Anything less than 10 is good, over that and you run the risk of throwing a flyer. 15 units or over is bad and should be used for foulers. In addition, the Berger's had more than a third of the sample with deviations less than 5 units, nearly perfect.

By contrast, the Scenars had only about 60% with less than 10 units, and almost 6% were over 15. Not a great yield for bullets that cost about the same as the Berger's.

I will be getting some 140 grain Berger VLDs, and give them a try.

2/3/07

I used a Forster inside neck reamer to remove the donuts from the Lapua fireformed brass. The reamer was held in the lathe chuck, which was running at about 200 RPM. The cases were held by hand, and fed onto the reamer. The cases were NOT sized prior to this reaming, as the reamer is sized to fit perfectly into a fired case neck. The results looked good under the stereo microscope, and the neck walls did not change thickness or have additional thickness variation based on before and after tube wall thickness micrometer measurements. Bullets would slide easily all the way into the case after this process. The donut appears to be gone. The cases were neck sized with a Lee collet sizing die after reaming. Range testing will show if the donuts are permanently gone, or just hiding.

2/10/07

Remington Brass Prep

Since Remington appears to have manufactured another lot of 260 Remington brass, and there were plenty at distributors, I purchased 500 units for fireforming. I didn't like the hassle of the inside neck reaming the Lapua brass, as well as the possibility that this process could cause excessive runout.

The 260 brass was prepped in the usual way, with primer pocket uniforming, flash hole de-burring, neck sizing with a Lee collet die, inside neck chamfering with a VLD tool, and outside neck chamfering.

Since the Remington brass is not quite of the same quality as the Lapua, I measured the neck wall thickness variation of every case using the shop made test fixture shown below:

The brass was sorted by thickness variation, and only those that were under 0.0015" total variation were selected, the rest were set aside. Only about 15 brass of the 500 were outside this tolerance, with the vast majority under 0.001".

Indoor Fireforming Experiment using Unique and Cream of Wheat Filler

I decided to try the method of fireforming brass using a light load of Unique pistol powder in a case, topped off with Cream of Wheat cereal. I used a dab of Crisco food shortening on the top to seal in the cereal. The idea was to get the brass to the final shape without the wear and tear on the barrel. Also, I wanted to be able to do this in my shop, and not have to go to the range.

The first trial was done outdoors, and it was quite loud. However, there was no monster jet of cereal to speak of, so I determined that a sturdy muffler would be safe and would be in order. I then took an old well water pressure tank that was waiting for a final trip to the recycle yard, and drilled a 2" hole in the top so I could insert the rifle barrel. I lined the hole with a piece of rubber heater hose to protect the barrel. I covered up the tank with an old moving pad for further damping. This muffled the report very well indeed, as it sounds only like a heavy piece of metal being dropped on the floor to someone about 50 feet from the shop. Passing cars were louder. Please do not try and contain this blast with a plastic bucket or something fragile as it is quite forceful. This tank is about 0.125" thick, and made to handle 150+ PSI water pressure. In addition, it is very voluminous, and the gas has plenty of room to expand. Any smaller volume would be risky. You try this at your own risk!

I initially started off with 9 grains Unique over a WLR primer. The case shoulders were not sharp enough, so I increased the charge to 15 grains. This formed nice sharp shoulders. The picture below shows, from left to right, an unfired 260 Remington case, fireformed with 9 grains Unique, and fireformed with 15 grains Unique:

After about 30 trials, I noticed an number of interesting things:

	If the charge was not high enough, some Cream of Wheat would be left in the case. Even after the charge was increased to 15 grains, an occasional case would have some Cream of Wheat left inside. This has to be mechanically removed after firing (compressed air, probe).
	Even with a 0.004" crush on the unfired case (typical Ackley headspace), the force of the firing pin was enough in a few of the trials to cause a misfire.
	In addition, it was found that even with the crush, some of the fired cases ended up short by about 0.005" to 0.010". The firing pin had enough force to push the case farther into the chamber than desired before igniting. This is bad since subsequent firing with full loads could cause case head separation.
	The rifle needed to be swabbed out in the chamber area, and a patch pushed down the bore about every 10 rounds or so. The Cream of Wheat made a bit of a mess in the action, so that had to be cleaned as well. If it is not cleaned, it bunches up near the case shoulder in the chamber, and the cases ended up with a dent where the Cream of Wheat was piled.
	The bore cleaned easily after all this. I used brake cleaner to dissolve the Crisco, it did a great job. The borescope showed that no change or damage to the bore occurred.

In short, I am not all that thrilled with this method. It is greasy, awkward, inconsistent in final case length, prone to misfires, and overall not any better than just loading and shooting. In fact, I need the practice, so I will go back to my time tested method of finding a good performing fireforming load, and shooting at 600 yards for fun.

Unique and Cream of Wheat Method Perfected

After the middling results described above, I started thinking hard about how to make this process work, and work well. The advantages are too great to not give it a good try, given the cost of barrels and bullets.

The methods used, and the results, can be found in a article that I wrote for the 6mmbr.com website, found here. Once the process was perfected, it was a snap to fireform new cases to the AI shape.