I have a number of AR-15s in various configurations for various purposes but when to comes to general field use with a 5.56mm/.223 semi auto rifle, I take one of my Mini 14s.
I have three of them that cover a pretty broad spectrum.
1) I have on of the early 180 series Mini 14s and IMHO it was superior to the other, (181 series through the early 580 series) pencil barrel Mini 14s in terms of accuracy.
I bought it with a scope mount attached in place of the rear sight, but opted to replace the rear sight with a Tech Sights rear sight. I also added a Choate Browning style flash suppressor/front sight, partly because it's a better front sight and largely because the barrel harmonics are much better with a bit more weight at the muzzle. In that condition it's a solid 2 MOA capable rifle with M193 ammo (55 gr FMJBT at 3250 fps).
The 180 series Mini 14s have much lighter slides as well as smaller gas ports, and that shows in the superior accuracy compared to the later series pencil barrel rifles. I suspect they went with the heavier slide and .080" gas port for greater reliability and durability with the full auto variants of the Mini 14 under field conditions, but it was a step backward for the semi-auto civilian version.
2) I have a 184 series Mini 14 that a previous owner cut down to 16.25", probably in an effort to improve accuracy, as that was a common thing to try back in the day. When I bought it, it was still a solid 5 MOA rifle, which was probably why the previous owner sold it.
However with a Choate flash hider, a SOCOM size Accu Strut, a .045" gas bushing, a shock buffer, and a Tech Sights rear sight it became a reliable 1.5 MOA 5 shot group rifle with the same M193 ammo.
A Choate ventilated handguard also helps keep the barrel cooler, but I still like wood and it now sports a wood handguard. It doesn't make a difference at reasonable rates of fire.
3) I also have a 187 series Ranch Rifle that was surplused by the NC Dept of Corrections. It had been set up for a scope and had an enormous muzzle device that was a combination front sight, flash hider and bayonet lug. It weighed about 6 oz and with it installed it shot about 8" low at 50 yards with the iron sights at the end of there adjustment. But it shot 3/4" groups at 50 yards.
Consequently, I gave it the same treatment as my 184 series Mini 14 and it produces the same 1.5 MOA 5 shot group accuracy with M193.
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One key point here is that all of my Mini 14s have 1-10 twist barrels, rather than the 1-7 twist that Ruger switched to before moving back to a 1-9 twist.
The 1-7 twist aggravates any issues with less than perfect bullets and is less than ideal for 55 gr bullets in general. The mismatch of ammo to barrel twist is one of the reasons shooters often report poor accuracy.
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Ruger made two major changes with the 580 series.
Most importantly they replaced their worn out tooling and the 580 series and up have much tighter tolerances.
In the middle of 580 series production they switched to a heavier tapered barrel that increases stiffness and improves barrel harmonics. They are reported to shoot 2 MOA out of the box - like the original 580 series before Ruger modified the design for the heavier slide and went with the larger gas bushing.
The 580 through five eighty whatever series rifles also benefit from a gas bushing and accu strut, but show less improvement than the pencil barrel Mini 14s.
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For comparison purposes I have a Colt SP1 as well as a Colt manufactured M16A1 upper half on an NDS 602 lower. Both of these "M16/M16A1" style rifles also shoot 1.5 MOA 5 shot groups with M193 ammo.
In a similar configuration, both the AR-15 and the Mini 14 produce similar accuracy, but I give the Mini 14 the nod for slightly greater reliability and better handling.
