What is the out of box accuracy on a m1a scout? I am thinking to buy one.

It depends on what condition it left the factory. If everything is properly fitted it should shoot about 2 to 3 MOA. Sometimes better with match ammo.

With a Burris scout scope I can shoot 1 moa at 100 yards, I believe its 2.75 power. With open sites it opens up a bit, but it my fault not the rifle.  Haven't gone past 100 yet.  

I think they are plenty accurate.

Only time I ever shot mine, I was doing pretty well with open sights at 100 yds (about 2-3 MOA).  Then I stepped out to 200 and shot a magazine full while my buddy was spotting me with a scope.  About the same accuracy (damn, that 4" Shoot-n-See target looks tiny sitting on top of that front post), but my final 3 shots were inside an inch just outside the 10 ring.  I stopped shooting and haven't shot it again.  Don't want to mess up my record.  A guy standing behind me offered to buy it then and there.   Turned him down.

My SAI scout is 2-3 moa with the right handloads, on the right day, when I'm in the right mood, wearing the right underwear, etc.... Most of the time 3-4.  Best 5 round group with handloads too hot to use was 1.5".