Thread: Fire forming 270 to 280ai

Greetings all,
A few notes on the AI cases follow. Parker Ackley developed and wrote about his AI cases back in a more innocent time when pressure testing was limited. The two most popular were probably the .257 Roberts AI and .30-06 AI. Later the .280 Rem AI was added. All of these cases are loaded to low pressure by the factories, the .257 Roberts especially so. Case forming was easy as the chambers were set for a slight crush fit at the base of the neck to allow factory rounds to be fire formed. The barrel had to be set back a thread to allow this be done. Huge increases in velocity were claimed achieved partly by increasing pressure to .270 levels and partly by going well beyond. As I said it was an innocent age, sometimes called the intrepid years of handloading. More recently the .280 AI has been made a factory cartridge which has provided some pressure tested handloading data. Out of interest I calculated the probable velocities for the .280 Rem at .280 AI pressures and found the gain to be minimal, in the 30 to 50 fps range. Other pressure tested comparisons for the .30-06 AI with its parent case showed the same thing. This does not mean that the AI cartridges do not perform well, they do, but the Ackley shoulder does not by itself provide vast increases in velocity as often claimed when all else is equal.
The .280 Remington has the shoulder set forward about 1.5mm from the other 06 based cartridges so the round can't be chambered in the .270. As both .280 Rem and .280AI cases may be hard to find this complicates forming appropriate cases. Both the .270 and the .280 cases are over 1mm longer than the .30-06 and other 06 based cases due to being based on the .30-03 cartridge, an earlier form of the .30-06. The .270 case will give the right case length but part of the neck will be formed from the case shoulder which may create a donut problem and the .30-06 case will wind up short. In either case there will be a grossly excessive headspace to deal with which will need a false shoulder, a jammed projectile or both. The C. O. W Cream of Wheat method with a fast powder and topped of with Cream of Wheat is an option as well. That 40 degree shoulder does look neat though. I had a .30-06 AI for a bit but never did much with it and it has long departed.
Regards Grandpamac.