It is my recollection, but a GI M14 had to shoot within 3.5 inches at 100 yards (or 100 meters) with GI ball ammo.
This was actually difficult to do as Ball ammo was not consistently capable of holding 3.5 inches. See bottom target.
This is mentioned in a Larry Moore technical report in the Collector's series book on the Garand to the M14.
Because I have no sense, I had a M1a built around all GI components, and then had the gunsmith bed the action, unitize the gas cylinder, add Garand era NM sights (no rear aperature, ½ MOA on windage but 1 MOA on elevation), and ream the flash suppressor. Still, it has that thin HRA barrel and that thin GI stock.
I have only shot the thing a couple of times, in a couple of 100 yard highpower matches. The first match, the front sight was loose and I got an oval group. Once I figured that out, I shot it again in another 100 yard match.
With the very light GI stock, and that light barrel, slung up in the prone slow position, this rifle is very twitchy. I found as I was shooting my 20 round string. Just tiny positional changes would send bullets flying anywhere. When I was finally getting used to the rifle, (and a goofy aimpoint with that 1 MOA elevation adjustment), I was consistently hitting in the ten ring as long as my position and trigger pull were perfect. But everything had to be perfect. The low and high shots are just me sliding up and down on that stock, or trying to figure out the post position.
I am not going to shoot this in any more matches, because it is a waste of my time. But I will bust rocks with it.
A M1a with the heavy barrel, heavy stock, is much easier to shoot and is not as sensitive to twitches as a GI configuration.
This target is 1968 WRA GI ball fired from a very accurate Ruger Tactical. How anyone could expect to shoot well with garbage ammo like this, is beyond me. But this would be what you would have been shooting in your M14 in Vietnam.
Incidentally, this is IMI FMJ's. These bullets are not so great either.
This is a 10 shot group with Hornaday 150 FMJ's. Which is darn good. If you shoot stuff like this, you might get a good idea of what capability your rifle has.