I posted this in another thread but thought it needed its own place.
The SIG P220 was a revolutionary pistol that very few people realize was revolutionary
Prior to the SIG P220 [AKA Swiss Model 1975], most pistols that used the Browning short recoil system utilized a pair of lugs on the barrel and corresponding cuts inside the slide to handle the barrel to slide locking. The SIG 220 used a new style squared off ejection port and a square barrel profile near the breach to handle the barrel to slide locking function.
That SIG style locking system has since been copied by countless other pistol makers. Glock, H&K, Ruger and many other designs now use that simplified locking system pioneered by SIG. It's easier to produce and fit the SIG style system, it's far less expensive to produce and it's very reliable.
When the Swiss were seeking to replace the P210 [AKA Model 49], SIG developed what became the P220 [model 1975] and that pistol incorporated several innovations. The square off ejection port barrel locking, the formed steel shell slide with a separate breach block, a new de-cocking system and separate removable steel locking block in the alloy frame. The P220 was originally designed as a 9mm pistol and the 45 ACP chambering was added later. The P220 has an excellent double action pull and still has one of the better DA to SA transitions.
When West Germany was seeking new police pistols the P220 became the basis for the shorter P6 (similar to a P225) pistol that was widely accepted by several West German states.
So the P220 is more than just a great DA/SA pistol chambered in 45 ACP, it was a significant milestone in handgun development.