Look at the basic AR which is a very accurate semi (or at least can be made to be). The gas port is some distance from the muzzle and gas is siphoned off. This changes the bore pressure and thus bullet thrust. Is it the same every time?
Doubt it. So you have a variation in the energy affecting the bullet and the barrel. It is not and cannot be identical each and every time so you have an ongoing variable that will eventually affect the launch of that bullet.
For semis that impinge on moving parts, this variable gets even larger as parts will never move the same so that creates all manner of changes in the vibration of the rifle.
Can you see very good practical accuracy, of course and there are plenty of semis that have honest sub MOA performance even at longer distances. Will this be able to match or beat the performance you can get from a well tuned bolt gun... even some factory rifles? NO.
While in Raton, F Open rifles routinely hammered out the center of a 5" X ring. I scored a shooter that for 8 shots wasn't much over 4" in group - the indicator disk we used was hammered with holes. The final score was 1 point dropped (not by much) with a high X count. For 17 shots, the group was well under MOA at 1000yds in the wind. That tells me the mechanical accuracy of that rifle was easily inside the X ring - sub 1/2 min. He had to hold off center to hit center given the turbulent winds.
True 1/3 min to sub 1/2 min accuracy at 1000yds in an Open and some FTR rigs is now pretty common and that is for up to 22rds relays. I doubt you will find a semi that can hit MOA at that distance ON AVERAGE, let alone allow a shooter to keep them in the 10 Ring over that many rds. when the air is bouncing around.
Jerry
