There are some tricks used by guys who build match grade service rifle uppers, but they may or may not be worth your time or of interest to you.
Joe Carlos is a retired US Army MSG who served as an armorer for the Army Reserve Marksmanship Team. He claims that in his experience, shimming and gluing a barrel into an upper will reduce group size by as much as 1/3:
https://www.youtube.com/watch?v=pJL-...efHKg&index=35
https://www.youtube.com/watch?v=pJL-...efHKg&index=35
(There are several more videos in that series with him, but the above it the most to-the-point about accurizing an upper by improving the fit between the barrel and receiver extension).
You could probably approach the press fit he talks about by using an upper that in manufactured to have a tighter than normal fit to begin with, as are BCM forged uppers IIRC.
Another short and less helpful video covering the same thing:
https://www.youtube.com/watch?v=y-jKERY7KJE
Criterion also recommends gluing their barrels in during installation:
https://www.youtube.com/watch?v=XHxU...efHKg&index=28
Again, maybe not for you, but it is something that some folks are doing.