"That can effect accuracy in a few ways. Increased lock time, the slow hammer fall gives you more time to move off target. Also in some cases it can cause inconsistent primer ignition.
A lot of people will dispute this, but I've seen it first hand with my own guns. Drove me crazy till I realized what it was. All my reduced springs went in the trash years ago because of it."
You can bet that your "action job" included a change in springs to a lighter main spring. I have never understood this need folks have to lighten the main spring on a single action, but that is what they do.
If you do a LOT of reading on gunsmithing and gun repair in general as I have over the past 30 years, you will find NUMEROUS stories of accuracy destroyed by lightening the firing pin strike on the primer. This includes bolt action rifles where the owner used grease on the striker and took it out in cold weather and accuracy went to poop. This may be what you did with the action job. Lightened firing pin strike on the primer. If the primer; especially the primers used in magnum ammo; are not hit hard enough and consistently enough, it will cause wide velocity variations, that may not show up on the target at 25 yards, but will at 100 yards. In addition, the slower hammer fall will allow more time between release and impact, and cause any lack of follow through on the part of the shooter, to be seen on the target.
In short, go back to a full strength main spring and this may fix your accuracy problem.
It is worth a try. If you sent your gun back to Ruger and it has light springs in it, it will not come back to you with those light springs. Ruger will undo that action job for you and get your gun shooting properly again.
For some reason, many of us think we know better about how the gun should be made than a company that had spent thousands or in some cases millions in research and development and built hundreds of thousands of them.
Best of luck
Louie