Quote History Quoted:
The Standard Model is the original Ruger introduced in 1949. Standard Models were produced until 1981.
The Mark I target model was introduced in 1950. It differed from the Standard in having adjustable target sights. Over the years, the Mark I target models came in various barrel lengths and weights. The 5.5" bull barrel eventually became the most popular. Like the Standard, the Mark I was discontinued in 1981.
In 1971, the grip frame on both the Standard and Mark I was changed to what is known as the "A 100" style. This meant that the magazine follower button went from being on the right to being on the left.
Newer Ruger replacement magazines can be assembled with the button on either side. Newer Ruger magazines are 10 shot while the original ones were 9 shot, but they can be used interchangeably in the early pistols.
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The tooling for stamping each side of the frame wore out and was replaced in 1971.
Ruger took the opportunity to move the button from the right side to the left side so that a bolt hold open device could be added at a later date. Ruger apparently had it on the long range radar when they re-tooled and created the A-100 frame.
Ruger also moved the Ruger emblem from the left grip plate to the right grip plate in 1971, so it's easy to tell the "old model" and "new model" pistols apart. The spacing on the grip screws also changed, so you can't interchange the old and new model grips.
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The post 1971 9 round "new model" Standard and Mk I "AMC" magazines will work fine in a Mk II, they just won't operate the slide stop.
The MK II "M-10" magazines will work fine in the post 71 "new model" Standard and Mk I pistols as is, and in the earlier "old model" pistols if you reverse the button - a process that takes about a minute.
However the original "old model" Standard and MK I magazines won't work in the "new model" pistols - standard, Mk I or Mk II.
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You can still buy Ruger factory 9 round magazines for the Standard and Mk I, but they are not the same as the originals. The originals had a silver finished Zamak magazine base and a slightly different magazine body. The current 9 round magazines are made like the later Mk II magazines with a black plastic follower.
The Standard and Mk I magazines did not have the Ruger emblem on the base plate - that feature arrived with the Mk II.
So don't lose the original magazine, it'll hurt the value of your pistol if you do.
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The Standard only came in 2 barrel lengths.
The RST4 had a 4 3/4" tapered barrel, while the RST6 was introduced in 1954 with a tapered 6" tapered barrel.
In general the Standard had fixed sights, but after the Mk I was introduced, Ruger offered the Micro adjustable sight as an option. Similarly, Ruger offered the improved Mk I trigger as an option, so you can find Standard pistols that are in effect tapered barrel Mk I pistols.
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The Mk 1 was introduced with a heavier, but still slightly tapered 6 7/8" barrel as the T678. In addition to the adjustable Micro rear sight and a more heavily undercut front sight, it included a modified trigger with stops to reduce over travel and reduce the slack in the trigger.
Ruger also made a short barrel Mk I, the T514, for a short time. This had a 5 1/4" heavy tapered barrel. It had target rear sights, but the front sight was not undercut as much as the T678 or the later bull barrel target pistols. Demand was low and it was discontinued.
The much more well know T512 was introduced in 1964 with the 5 1/2" bull barrel. The front sight on these is attached via 2 screws rather than with a band around the barrel. The "Micro" marked sight went away in 1969, replaced by an un marked adjustable target sight.
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The Mk II added the beveled rear receiver as well as a better trigger, a bolt hold open device and the 10 round M-10 magazine.
It came in two "Standard" versions with a 4 3/4" barrel (catalog number MK-4) and a 6" barrel (catalog number MK -6). This expanded to 4 models in 1984 with the stainless KMK-4 and KMK-6. Unlike the earlier "Standard" pistols, all of these were sold with fixed sights, and there was no adjustable sight option.
The Mk II Target Model also came in blued and stainless versions as the MK 678 and KMK678.
The MK-512 5 1/2" bull barrel target model was introduced in 1982, the MK-10 10" model was added in 1984 with a slightly tapered heavy barrel and the MK-678-G 6 7/8" Government Model with a slightly tapered heavy barrel was introduced in 1987. The 5 1/2" and 10" models also came in stainless versions as the KMK-512 and KMK-10.
The MK-678-G had 1-15 rifling and was accuracy tested at the factory and had to meet or exceed 1" at 25 yards.
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Below is my 1981 vintage T678 Mk I Target and my 2002 vintage MK-512 Mk II Bull Barrel. Both weigh nearly the same (40.8 Oz for the T678 and 40.2 oz for the Mk-512). I like the longer sight radius on the T678, but the MK-512 balances better.