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. Recoil Calibration Pak contains 1 each of seven different springs; 16½ lb., 15 lb., 14 lb., 13 lb., 11 lb., 10 lb. and 9 lb., plus three extra power firing pin springs. Individual Spring Pak contains one variable power recoil spring and one extra power firing pin spring. Factory weight is 16 lb.
1911, Issue:11, Page:033
Big Book Catalog, Issue:68, Page:132
Sinclair, Issue:1A, Page:031