Over the last thirty years or so building rifles and reloading, one subject that comes up frequently are the Ackley improved or more to the point ,what is the correct way to form brass. I'll get into some details and ramble on a bit, but I think in the end it will make sense.
( NOTE...I'm neither a writer or good picture taker. The old saying "A picture speaks a thousand words" applies .....)
The design is such that factory or handloaded ammo can be safely fired in an improved chamber. In reality,they are not ture wildcat cartridges. Wildcats are considered a rifle for which no factory loaded ammo exists. A 6.5-06 would be an example. The 6.5-308 was a wildcat for many years until Remington standardized it,now known as the 260 Remington. The 7mm-08 is another example. The term "improved" is a round reconfigured by fireforming,but where both ammo and chambered rifle exist.
With that, lets look at a typical AI cartridge. First, the shoulder is typically moved to a steeper angle, 40 degrees the most common, second, the body taper is reduced to a minimun spec,normally .010" case web to the shoulder and third, the neck-shoulder junction is moved back. This is a picture of a formed AI and parent cartridge, a 22-250 and 22-250AI.
The method to form AI brass is actually quite simple by firing factory or handloaded ammo,but I need to make a few comments first.
We all hear about the term "Headspace". A black art or a mythical process to some, but in my opinion, its simply a piece of brass trapped between two unmoveable pieces of steel. I believe in the K.I.S.S. principle.
Headspace is normally the distance from the boltface to a point on the case shoulder known at the "Datum" for a bottleneck rimless cartridge,however there are other ways to acheive it. As an example,when the 204 Ruger hit the market factory brass was at a premium, more costly than gold I think. One work around was to reform 222 Rem. Mag brass, the parent of the 204 R. Since I had a supply of left over 222 RM brass,by partically resizing the neck in the 204 die creating a "false" shoulder and fireforming, I made 204s. My load for the 204 is 29.5-BLC2 with a 32 grain bullet, the forming load,28.0-BLC2 and a 32. As to why that forming load I'll get to later. Take notice of the shoulder position between the 222 RM and 204 cases.
Another method of headspacing as used to form AI brass is the neck-shoulder junction. The design of a true improved places the junction back from that of the parent round. The reason for that is when a factory case is chambered the only contact is with the boltface and the junction. Remember, the AI chamber has a straighter body and steeper shoulder angle, so the casehead and shoulder-neck junction points are the only place a factory case will make full contact. As a side note, the orginal spec for the 30-06 was the same, casehead to neck-shoulder junction, not the datum!
Here are three examples, a 22-250AI,250-3000AI and a 270-308AI next to the parent cartridge. All three parent cases were randomally selected from lots of brass, right out of the bag except the 270,which was necked to 27 from 308 brass only.
Notice,in all three examples, the location of the neck-shoulder junction.
Here are the three parents,smoked to show where they headspace in the AI chamber. The bright ring indicates contacts the chamber wall or zero headspace.
Next, the parent, a chamber cast and a formed 250-3000AI case.
The pictures show the correct method for headspacing a factory parent to form improved brass. When the factory round is chambered there will be a slight crush fit if the chamber was cut correctly.
Now, if the chamber was cut correctly and is in fact a true AI, there are two way to make brass. One, simply fire factory loaded ammo of you choice, the other, handload forming rounds. Now, regardless of the opinions,thoughts,ideas and rumors what charge to use, the correct one is a stiff charge for the parent round. As an example, for my 22-250AI I shoot a load of 39.5 - Varget, the forming load is 36.0-Varget, both with a 50 grain bullets. That 36 grain load is just under the top for a 22-250 listed in the Sierra manual.
Although some suggest and get away with a reduced load to form brass, from expericence, I don't recommend it. A reduced load may casue "bouncing" effect and not fully form the case. (P.O. Ackley words) Although appearence wise it looks perfect, they can be as much as .030" short causing a casehead or top shoulder seperation. In my 30 some odd years using AIs I have seen this time after time. About a year ago, a showed me a case with a complete shoulder seperation from a 243AI. It happend on the second firing He had formed the using a light load for the 243 Win. Measurements of the formed brass showed they were .005 - .0015 short. I made a chamber cast to confirm the chamber dimensions, loaded a new batch of ammo and they formed exactly right. Lesson learned.
Another miscomception is a bullet jam. A bullet will not provide headspacing, it moves! Remember my defination of headspace, a brass case between two "unmoveable" pieces of steel? A bullet will begin moving before the case can expand 100%. I've heard of the firing moving the case forward when the primer is struck. If this happens,the case is too short, or the chamber is too long. I'll say it again, "a factory case will headspace on the neck-shoulder junction in a correctly chambered barrel".
I continue mentioning "correctly" chambered.. There are those that simply re-ream an existing chamber to convert to an AI without setting back the barrel. This creates a chamber longer than a true improved. Remember the AI is longer from the casehead to the top of the shoulder and shorter at the neck-shoulder junction than the parent round. For a 22-250 - 22-250AI the measurements are....
Casehead to shoulder ............ 1.514" ... 1.524"
Casehead to neck - shoulder .. 1.664" .... 1.644"
Overall length ....................... 1.912" .... 1.892
Top shoulder diameter ............ .414" ..... .454"
Using these measurements, you can see why setting back the barrel is required. If you thinking of having the conversion done,ask how the job will be done . If it doesn't include the setback, find another 'smith...quick! I made that mistake with my first one,a 280AI. Another gunsmith did the job over, correctly.
At times a reamer is cut to a special specs, ie..freebore,neck diameter,etc. Ask for the reamer specifications before having the job done. Its always a good idea to have the 'smith test fire the rifle and supply a few fired cases. The last few I've put together have been fully threaded and chambered Savage barrels. To headspace them, I use a factory unsized case. The 22-250AI and 250-3000AI in the pictures were done using factory brass and fireformed. In fact, all my Savages have be setup this way, AI or otherwise.
After forming, reloading for an AI is exactly the same as any other catridge and caution should be used in working up a loads as one would normally do.
Now, some may agree,disagree or have other opinions This is what I've been taught, do and have formed thousands of rounds without a problem.
Well, I hope this helps those thinking the AI route. They are not the problem some make them out to be. The benifits are a higher velocities and extended case life if loading is kept within reason, but one more step in the process of reloading.
Take care !