To fireform 257 Roberts Ackley that headpaced like a 257 Roberts. Not 0.004" shorter, like I should have done:
1) Form half a shoulder with 10 gr of any pistol powder and covered with cream of wheat. No bullet. Fire straight up if allowed. To fire horizontally at the range, cap the case mouth with Ivory soap so the powder will not fall out. This half shoulder is adequate to stand up to the firing pin push. Without this step the case can stretch and crack just behind the shoulder.
2) Load and fire with full power 257 Roberts Ackley load and expect usual accuracy.
3) The shoulder is now fully formed.
