Variable rate springs function correctly with a wider range of loads than fixed rate springs can. Rate increases as spring is compressed; gives greater protection against frame battering, more consistent lockup and improved feeding, while allowing easier manual cycling of the action. All springs are variable power. 18 lb. spring is factory standard; includes extra power firing pin spring.
1911, Issue:09, Page:032
Big Book Catalog, Issue:66, Page:122
Sinclair, Issue:1A, Page:031