Key difference: The first gun of the series that was launched by Ruger was the Ruger Standard Model .22 Auto. As the series progressed, this model was often referred to as Ruger MK I. The Standard Auto was followed by Ruger MK I Target, then the Ruger MK II and the current Ruger MK III and its variants.

The first gun of the series that was launched by Ruger was the Ruger Standard Model .22 Auto. As the series progressed, this model was often referred to as Ruger MK I. MK stands for Mark, which is used to denote a change. The Standard Auto was followed by Ruger MK I Target, then the Ruger MK II and the current Ruger MK III and its variants.

The Standard Auto was first launched in 1949. It was based in the Nambu and Luger designs. It came with a blued carbon steel finish and was equipped with a 4.75-inch (12.1 cm) tapered barrel. It had a cylindrical bolt that cycled inside a tubular receiver in a manner more characteristic of a rimfire rifle. The bolt of the pistol had protruding “ears” at its rear. The Standard Auto featured a basic blowback form of operation.

The magazine held 9 rounds of .22 Long Rifle ammunition. The gun held the magazine in place by a catch on the bottom of the grip frame. The Standard Auto also had a Patridge style fixed iron open sights with the rear sight securely mounted in a dovetail. Also, the manual safety on the gun could only be engaged when the pistol was cocked. In addition, the bolt could be locked open by activating the safety with the bolt held back.

The Standard Auto was finally discontinued in 1981. The MK II was introduced the following year, 1982. In the MK II series, the slot for the magazine follower extension on the grip frame was on the left side as opposed to the right in the previous models. This lead to the addition of a bolt hold open device on the MK II. The bolt hold would hold the bolt open after the last round in the magazine was fired.

The MK II also featured twin scallops, or recesses, at the rear sides of the receiver to allow easier grasp of the ears of the bolt when cocking. The magazine of the MK II also held 10 rounds of .22 Long Rifle ammunition, instead of the previous 9. Furthermore, the safety was redesigned so that it locked only the sear instead of both bolt and sear. This allowed the bolt to be pulled to the rear for visual inspection of the chamber, in addition to the loading or unloading while the safety was engaged. The MK II also featured a better designed trigger pivot retainer. It had a music wire spring as opposed to the original use of a lock washer. This made it easier to disassemble and reassemble the gun.

The MK II was eventually removed from production in 2004. It was then replaced by the MK III and its variants.

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