By contrast, it is not hard to build an AR that will routinely shoot 1/2 MOA, and some will routinely shoot even better. jpwilly is dead on. M1A are basically non-existent in service rifle matches. They can't keep up with the ARs.
All True and partially unfair.
As someone who got their last leg and their Distinguished with a M1a, well I have a lot of fond memories of the "stick".
I have shot non matched out M1a's, and 2 MOA would be fine with one of them.
However, to get the maximum accuracy from a M1a, it has to be totally matched out. Glassbedded, gas cylinder unitized, flash suppressor reamed, and there are probably some other modifications that have to be made. This is still done by a very few guys, and it costs mucho dinero.
A totally matched out M1a is capable of sub MOA groups, which is pretty darn good. And regardless of what you read on the web, it is very hard to shoot sub MOA off the ground, in a sling, with iron sights, in a match.
The Mouse gun has been built into a reliable and accurate match rifle. Because of the lesser recoil it is far easier to shoot. This is very important as you pick up points, and X’s in the rapid fire stages. At 600 yards the M1a with a 175 might have a slight ballistic advantage over the 80 grain .223 (always debatable), but what ever points you might think you will get long range with the big bullet, you will actually earn less than the ones you lost on the short range.
It is true, the average NM AR will shoot inside the average match M1a, but not by much. And there are few shooters around who can actually demonstrate this on paper. There are about a million more who think they can.
I was there when the Marine Corp made the change from M14 to NM AR. So I asked the guys, “how are you doing”?. The answer was "same offhand, better in the rapids, and worse at long range". But that better in the rapids made all the difference.
I have attached my highest X count sitting group target with a M1a. (100-8X) Also attached a 100 yard reduced course target standing. Shot recently. Not the first 99, but never have got that 100. I have cleaned the 500 yard target, never cleaned the 600 with a M1a, but by the time you get in the pits usually your target has been pasted over, such is life. The only targets I get to keep are the reduced course ones, and the photo of this 200 yard sitting.