In a shorter 16” barrel the 5.56 is more accurate. So an AR is going to be more accurate than an AK. But if you do some work on an SKS with a 22” barrel it will be more accurate than either with a 16” barrel because of more pressure build and velocity. I can out shoot either with my SKS and 7.62 hunting rounds. So the rifle absolutely has a lot to do with how accurate a certain round can be. And so can the brand and weight of the bullet.
Well, yes. There are so many variables that one could go on for hours about it. Barrel length, twist, type of powder (fast/slow), Bullet weight, air temperature, phase of the moon. So many things effect a firearm’s performance that it’s difficult to make blanket statements.
SKS barrel is 20”. I’ve shot at clay at 400 plus yards with an SKS and had the bullet penetrate about 1/2 its length.