I have shot high power with numerous rifles -- service and scoped. As I recall the X-ring is 7". At 600 yards that's a little over 1 MOA. I've yet to hear any one who shoots all Xs. The 10 point bull is 13" which is slightly more than 2 MOA at 600 yards.
My sniping instructor often reminded us that 2 MOA in the field is realistic expectation.
The X ring of the MR target is 6 “. Just under a true MOA (1.047 at 100yds).
As I stated, If I do my part. Service Rifle (AR 15 223/556) 600 NRA record is 200-17x if I recall.
My PR is quite a bit less- 197-9x My best overall group of 20 record shots was a measured 1.8 MOA or ~11”
Of course, HP is done under controlled conditions with or w/o sighters and dedicated custom ammo, heavy coat, sling, glove etc and match components. It is not uncommon for AR SRs to shoot bug hole groups, but the SR and MR targets are quite generous, 1 MOA X ring, 2 MOA 10, 3 MOA 9 etc
With the allowance of up to 4.5 power optics, the scores have crept up a few notches overall.
An as issued AR15 A2 or A4 with ball ammo is not even close to being competitive- they are generally at 2-4 MOA combination and a sling degrades it from there due to non floating HGs and barrel.
The little AR15 223 can be quite capable at range.