Switching to an after market barrel will give you the greatest increase in accuracy of any. I would call the magnitude as significant. I routinely take 1.5 MOA rifles and turn them into .5 MOA weapons with a barrel swap.

I've use Shilen, Lilja, Krieger, and Douglas barrels. I've been getting excellent results from Douglas. Truth is I use Douglas exclusively now. I love Shilen barrels but for the price difference their just isn't much difference in a Douglas.