Primer Pocket/Folded Head
Based on design experiments by Col. Hiram Berdan, the folded-head case was developed.
Folded head case
Image courtesy of
International Ammunition Association Journal
A central pocket for a separate primer and an anvil surface were produced to provide the pinch point that activated the priming compound when the firing pin struck the primer. One to three holes were pierced or drilled around the anvil to allow fire from the primer to reach the main charge. Other than the dimensions, the primer was similar to the familiar percussion cap of the muzzleloading era.
The folded-head case had a weakness; the fold that produced the rim stressed the metal. Over time, the stressed area became brittle, causing the case to fail at the fold. The metallurgical knowledge to properly relieve the stress in this area was in early stages of development; some batches of cases were loaded and issued with brittle rims.