260 Ackley Brass Fireforming

Brass Selection, Brass Preparation Techniques, and An Alternate Method for Fireforming

Background

The 260AI rifle project was started after shooting a 6.5-284 for a couple of varmint hunting seasons, with the goal to able to equal the larger cartridge's external ballistic performance with less powder, more efficiency, and hopefully more barrel life. The initial tests were performed with a end-of-life 3 groove 9" twist Pac-Nor barrel taken off the 6.5-284, then set back and chambered with 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.

As this project unfolded, a number of lessons were learned regarding brass preparation and methods of fireforming the Ackley Improved cases, as well as issues induced by using necked up .243 Lapua brass.

Standard Remington 260 brass was used during the initial testing, and 100 cases were screened by measuring the neck wall thickness, and discarding any that exceeded 0.0015" total variation around the circumference of the neck. Out of the initial batch of 100, 83 cases met this criteria. The primer pockets were uniformed with a Haydon carbide uniformer tool, and the flash holes were deburred using a Haydon flash hole tool. The necks were sized in a Lee 260 Remington collet die, then inside and outside chamfered. The cases were then primed with Federal GM215M large rifle match magnum primers.

Fireforming was performed using the tried-and-true method of seating a bullet past the jam point, with good neck tension, so that the case would be held firmly against the bolt face upon initial firing. The load used was 47.4 grains Ramshot Hunter, under a 142 grain moly Sierra MatchKing. This load resulted in velocities around 2950 FPS, and accuracy at about 0.5 MOA for 5 shot groups. There were no split necks out of this first batch of cases, and the fired case length was very consistent. The length was measured using a Stoney Point case comparator on a pair of digital calipers, with the length variations under +-0.001" for all the 83 cases. The brass had a water capacity of 57.4 grains after fireforming. More importantly, there were no donuts formed inside the case at the neck and shoulder junction point.

Switch Barrel Project Rifle - .243 Win. Lapua Brass Preparation and Fireforming

Based on the positive preliminary test results, a 29.5" Bartlein 5R, 8.5" twist, 1.25" straight cylinder contour barrel was chambered with the 6.5-243 Improved reamer and fitted to the trued Remington action and Lowrider stock that is the platform for a very accurate 6mm Dasher rifle. This action has a pinned recoil lug for Holland's, allowing easy field swapping between the 6mm and 6.5mm chamberings.

Due to the lack of availability of 260 Remington brass from Remington at the time of this second set of tests, the decision was made to use neck expanded Lapua .243 Winchester brass for barrel break-in and long range load work up. It was felt that the high quality and intrinsic consistency of the Lapua brass would contribute to a superior long range load.

Brass preparation was identical to the process used on the original Remington brass, with the addition of a neck expansion step prior to the neck sizing and deburring processes. A 6.5mm expander mandrel from K&M, aided by a small amount of Imperial Die Wax was used to bring the necks up to the proper inside diameter.

Fireforming of the .243 Lapua brass was again accomplished using jammed 142 Sierra MatchKing moly bullets, over 47.4 grains Ramshot Hunter, and a GM215M primer. Accuracy was excellent, with less than 0.5" five shot groups at 100 yards the norm. As with the Remington brass, no split necks or burst shoulders were observed. The case water capacity of the fireformed Lapua brass was a bit less than the Remington, at 55.9 grains. This is no doubt due to the heavier construction of the Lapua brass.

After fireforming, the Lapua brass showed a pronounced internal restriction at the neck shoulder junction, thick enough that a bullet would not pass. This restriction was measured to be 0.005" smaller than the fired neck inside diameter using a Starrett hole gage.

The formation of these donuts is, unfortunately, a common occurrence with some cartridges. Many manufacturers, including Sinclair and Forster make a special reamer specifically designed to remove the donut from the insides of the neck.

Although Forster designed these reamers to be used with their neck trimming tool, it was decided to try using the reamer by holding it in the chuck of a small lathe, with the rotation speed set at about 200RPM. The Lapua cases were held by hand, and carefully fed onto the rotating reamer. The as-fired cases were not resized prior to this operation, as the reamer was sized to fit perfectly into a fired case neck.

While the results look promising under a laboratory grade stereo microscope, there was some concern that excessive material may have been removed from the neck walls higher up than the donut, causing undesired wall thickness variations. It was decide to leave these cases for further evaluation at a later time, and concentrate on preparing a newly acquired lot of Remington 260 brass for fireforming and load development.

Remington 260 brass again became available shortly after the Lapua donut issue was resolved. Since the Remington brass was simple to prepare for fireforming, showed good results in the initial tests, and the Lapua brass was suspect after the inside neck reaming, the decision was made to proceed with the project using a lot of 500 Remington cases. The new 260 brass was prepared using the same process described previously, 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.

Again, the neck wall thickness of every case was checked, and any case showing more than 0.0015" maximum variation was set aside. Only about 15 cases out of 500 failed this test, with the vast majority under 0.001". In all, the Remington brass was very consistent, with were very few burrs in the flash holes, and most primer pockets had little or no material removed during uniforming.

As before, the wall thickness variation measurements were made using a shop made test fixture shown below, but other vendors, like Sinclair International, sell a similar instrument.

Others have documented successful fireforming of Ackley class cases using light loads of pistol powder which drives an inert filler material into the case shoulder hard enough to fully expand the brass into the desired shape. There would be multiple advantages to using this technique; as there is no bullet ejected, only the filler and some high pressure gas, the cost is low since no bullet is required, and there is a good chance that the barrel would incur minimal wear.

After quite a bit of web searching, very little concrete documentation relating to the actual details of the process was found, so it was decided that some effort was to be put into developing a safe and effective process. Based on what little information was available, it was decided to use Unique pistol powder, a Winchester Large Rifle primer, regular (not instant) Cream of Wheat breakfast cereal as the inert filler material, and a dab of Crisco vegetable shortening at the top of the case mouth to hold it all in during the firing process.

The initial load was 9 grains of Unique, with the case filled to within 0.125" of the top of the neck with Cream of Wheat. A small amount of Crisco was then placed on top and smoothed with a finger to seal it in. A 10cc syringe was packed with shortening and used like a cake decorating tool to apply the grease evenly and neatly.

Not knowing what to expect in terms of report and force of the blast, the initial test was done at the range following standard range safety procedures. The report was quite loud, about like a starter pistol or a 22LR. However, other than a jet of gas and vaporized cereal, there was little recoil, and no projectile or lump was observed to be ejected. Even so, it was obviously a very violent event, and not to be taken lightly.

The starting load of 9 grains Unique did start to form the Ackley shoulders, but there was obvious incomplete expansion into the very corners of the new chamber. When the charge was gradually increased to 15 grains, nice sharp shoulders were consistently formed . The picture shows, from left to right, an unfired 260 Remington case, a case fireformed with 9 grains Unique, and a case fireformed with 15 grains Unique.

During the first 30 trial shots, about 5 misfires were observed, along with a number of fired cases having a significant amount of filler left inside. This remaining filler caused a mess by spilling all over the inside of the action, requiring a cleaning by blowing out the action and chamber with compressed air. In addition, the chamber (and the operator's hands) would get a bit greasy from the Crisco, also requiring a bit of cleaning with a solvent patch and a rag. Brake cleaner worked great in cleaning the bore after a few rounds.

The misfires were attributed to the forcing of the case forward by the firing pin strike, enough that erratic or no ignition would sometimes occur. The 260 Remington brass did not have enough mechanical strength to resist the firing pin blow, even with the standard Ackley 0.004" crush headspace setting. The results, while promising, required another trip back to the drawing board to resolve the final misfiring issues.

One of the most common methods to prepare 6mmBR cases for fireforming into 6mm Dasher (6mmBR Ackley improved) is to introduce a false shoulder into the case neck, just thick enough and long enough to firmly hold the case in place in the chamber during the firing pin strike. In the case of the Dasher process, this is done by expanding the 6mm neck with a 25 caliber or 6.5mm mandrel, then neck sizing with a bushing die, setting the bushing such that only a part of the neck is resized. The position of the bushing is adjusted until a firm force is needed to close the bolt on a prepared case. This locks the case in place, and assures good ignition and consistent case lengths after fireforming.

For this experiment a 7mm mandrel was used to expand the neck. The neck was then sized down using a Redding type S bushing neck die with a 0.292 bushing. The bushing length was set so that a very firm push was required to chamber an empty case with this false shoulder. The picture shows an un-modified case on the left, a case with a false shoulder formed in the middle that has not been chambered, and a false shouldered case that has been chambered and extracted on the right. The close-up photo clearly shows the region of the case that has been forced into the neck part of the chamber as a shiny ring at the base of the neck. This crush firmly locks the case into place during firing.

To illustrate the technique used to measure the position of the false shoulder during this trial and error process of adjusting the bushing, the following pictures show the use of a Stony Point 30 caliber bullet comparator as a nifty length gauge.

The close up clearly shows how the comparator rides over the 6.5mm case neck, and comes to bear on the false shoulder. For the test rifle chamber, a length of 1.800" provided the perfect amount of crush.

Once the cases were prepared with this false shoulder, there were no more misfires, and only 3 cases out of 100 fired had any filler remaining inside. This was a clear indicator that the weak firing pin strike due to case movement resulted in erratic and incomplete ignition as well as the misfires. This poor ignition was also the source of the incompletely ejected filler. No barrel wear was observed through a Hawkeye bore scope after cleaning with solvent and patches. All 100 case lengths were within +-0.001" after fireforming, and had very sharp shoulders.

Once the problems with case movement and erratic ignition were worked out, this projectile-less method proved to be very effective, and appears to offer a significant savings in both cost of components and barrel wear during fireforming.

07/16/16

Background