After reading all the posts, I have to put my two shillings in. Some factory guns are well made and shoot good in the hands of a marksman. And no matter how well made a gun is, if the shooter can't hit a bull in the a&% with a bass fiddle, it's worthless.
Since I've started shooting chunk gun matches, I've paid more attention to one thing than I ever have before: Consistency. This goes for all types of shooting. You take out as many variables as possible. This is where the custom barrels and locks, triggers and fitted stocks play a big part. When you marry the proper twist and rifling depth with your load, have a good lock that consistently throws fire or busts caps, a trigger that trips the sear without causing pull-off, you have eliminated many of those variables. And just as important, the fit of the stock and the sights. If the stock doesn't fit the shooter, he's likely to be uncomfortable and actually straining just to get his eyes lined up with the sights. And he needs to have his sights properly spaced both from his eye as well as from each other to get a proper sight picture. And lastly, many shooters believe they have to have a heavy load to do the job. That may be alright when hunting grizzlies, but not targets (except maybe very long range). Here you have to deal with recoil which will definitely cause anticipation at each shot and will cause flinching.
So, in other words, you may shoot good with a factory gun, but you'll shoot gooder with a custom gun. Without major modifications you can't meet the above requirements with a factory gun. If you do these things to an off-the-shelf rifle, you no longer have a factory gun, but a semi-custom.
That being said, it still boils down to the shooter no matter what he shoots. You can reduce all the variables with your weapon, but you still have to deal with the flaws or variables with the shooter. Like any other sport, you have to condition yourself and learn all the skills of shooting, reading light and wind, consistent gun handling, &c. I think this is harder than getting my weapon ready. When I shanked a shot the other day I said that there was something not right with my rifle. A buddy of mine said that it was the loose nut behind the trigger (meaning me).