If you "full length" resize your fired cases by adjusting your resizing die "out" until you can "Feel" the bolt close on your loaded round, you essentially have "Zero" or minimum headspace. You may want to go to a little "tighter" or "heavier" feel. Be careful not to over do it and not be able to close the bolt at all. If you set your resizing die to provide "Zero" or minimum headspace, setting the barrel back and rechambering cannot provide "tighter" headspace. (If it is any tighter you cannot chamber a loaded round.) Have you had the headspace checked by a "good" gunsmith with a set of "known" "caliberated" gages for the caliber? If you cannot get the accuracy with the ammo sized to provide "zero" or minimum headspace, I do not see how the accuracy problems can be related only to headspace. It is also possible that your resizing die is the type that should be "adjusted" rather than be set so that the shell holder "bottoms" on the die. This was not uncommon on some of the older dies and on some "custom made" dies. The chamber could be improperly cut, not concentric with the bore, be oversize on the diameter or have an oversize neck, or throat, or have a throat that is too long. It is also possible that the barrel is not properly torqued into the receiver or that it has the wrong rifling twist, is not straight, is badly fouled, has a bad bore or a damaged muzzle (crown). There are other possible problems as mentioned by others. I would thoroughly clean the bore, check for straightness, rate of rifling twist, and for any muzzle damage. I would then try loading some ammo to specifications known to produce accuracy in your caliber, in other rifles, (not reduced loads, as they can be variable) and to provide "Zero" headspace in your rifle, and see if that improves accuracy. If you wish to sell the rifle it is a very good idea to have the headspace checked, and if necessary adjusted properly, before selling the rifle. Hope this helps. Good luck.
