They come in three basic flavors.
1) The original Mini 14 was the 180 series. It was almost an entirely different animal, with a lighter slide, a slimmer gas block and stock, and a different, external bolt hold open. It was light, handy, and 2 MOA accurate. Jeff Cooper liked it and Ruger should have left it alone. Instead Ruger developed the AC-556 and then revised the Mini 14 based on parts commonality.
2) The 181 and later pre 580 series were based on the AC-556 with a heavier slide, which drove the requirement for a deeper gas block and stock. They also over gassed the, with an .080” gas port bushing (compared to .052” on the 180 series).
The extra reciprocating mass and over gassed condition inherited from the AC-556 combined with the slim barrel didn’t do good things for the accuracy. 3-5 MOA accuracy was the norm and if anything it got worse over the years as the tooling aged.
3) Ruger re-tooled before restarting production with the 580 series. Mid way through the 580 series they changed to a heavier tapered barrel. Between retooling, the towered barrel and a smaller gas port bushing they once again produced 2 MOA accuracy.
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The 181 and other pre-580 series can be made to shoot well with a few additions:
- an AccuStrut to improve the barrel stiffness;
- a Choate flash hider (either Browning or M16 style);
- an .045” gas port bushing; and
- a Tech Sights rear sight.
I have a 180 series (middle) as well as a 184 (top) and 187 (bottom) series. Both the 184 and 187 series shoot 1.5 MOA 5 shot groups with those changes.