The SR-series pistols came first starting in 2007. They were Ruger's striker-fired replacement for the previous P-series lines of pistols. The SR-series was apparently discontinued as a regular Ruger production line in 2018, but models have subsequently been produced for specific distributors.
The American pistols were reportedly designed for the military pistol trials. Ruger did not enter the pistols in the military trials and they were introduced in the commercial market in 2015. The American pistols are often considered to be more robustly designed than the SR-series pistols.
The Security 9 pistols, with an internal hammer-fired design, were introduced in 2017. Those pistols are generally recognized as an economy line that parallels economy models in other Ruger lines.
Here is my take on what happened:
The SR-series was a profitable line, but Ruger couldn't resist designing a pistol for the military trials. When the prospect of a military contract did not work out, Ruger had two fairly similar lines of pistols with similar price points. Awkward! Even worse, the new American pistols apparently did not sell very well. Finally, Ruger had already begun moving other lines to an economy model, which was embodied in the new Security 9 pistols.
By the end of 2017, Ruger had a slow-selling American premium line, an old SR-series premium line, and a new Security 9 economy line. The SR-series was discontinued, leaving one premium line and one discount line. Things went a little sideways when the American pistols were heavily discounted by some sellers, even to the point of approaching the prices for the Security 9. Things went further sideways when Ruger resumed making the SR-series as Distributor Exclusives and the distributor priced the pistols in the same range as the discounted Americans or the Security 9. In short, you can now buy pistols from any of the three lines for around $300 or less. That has to be a marketing nightmare for Ruger, competing against themselves.
