Why?
The original mainspring of the early 1st Generation Colt SAA was a whopping 0.127″ thick at the base! By the end of the 1st Generation production, Colt reduced the thickness to 0.080″. Over the last 148 years, the mainsprings have been reduced to less than half of the original — typical mainsprings on modern Colt SAA revolvers run 0.053″ thick.
So why was the mainspring so stout at first? Most likely it was due to the type of ignition used by the original .45 Colt cartridges.
For roughly the first 10 years, the cartridge used a Benet primer. The Benét primer looked like a rimfire cartridge but was a center-fire with the priming compound and anvil crimped inside the cartridge base. The firing pin had to hit with enough force to ignite the primer through the copper case. The Benét primer wasn’t replaced until the case design changed in 1882 to an external primer. We can assume the military had stockpiles of Benét primer ammunition to use up for much longer. Mystery partially solved.
John Kopec, a well-known authority on all things Colt SAA and co-author of A Study of the Colt Single Action Army Revolver, shed further light.
In the Colt records, regular and extra-strong mainsprings were sent as spare parts to the U.S. Army. Under what circumstances a mainspring would be replaced with an extra-strong version is not known but the strong mainsprings caused many of the soldiers to develop their own throwing down technique, which in turn causes problems.
The act of throwing down is violent and creates a great deal of energy in moving the cylinder. The original Colts had almost no bolt approaches and shallow bolt-lock slots. With the accelerated speed of the cylinder, it was common for the bolt to not lock in place and the cylinder to stop out of alignment. At best, this caused lead shaving as the bullet jumped the barrel gap. At worst, the revolver blew up. This happened enough the U.S. Army sent letters to Colt asking the design be changed. In the correspondence, this phenomenon was called “throwing past.” This can still happen today when gunsmiths lighten the bolt spring too much.
