Dude, for the life of me I can't remember the name of the company, but in MS there is a shop that specializes in Mini's.
They will either re-barrel the action or do a "trim" job where they remove a small amount of the stock barrel and recrown it etc. They claim that the manufacturing process that Ruger once employed, left the muzzle at less than optimal for accuracy. If I recall right they will inspect the bore and the crown and then contact you with the findings before they do anything. You will have to send the entire action so they can set everything right. Since the article is rather dated, I'm certian the price has went up. But the price noted then was more than reasonable for the work performed and it was cheaper at the time than what Ruger listed for similar work. They also have trigger options too. But depending on what you use it for, some may be awful light. The author mentioned "doubling" and "tripling" off the bench at first with the lighter trigger job.
This was in an article several years back in GUNS magazine, so you may be able to hit their website and do a search to find who the company is. They claim to have done wonders with older Mini-14's.
Also, Ruger used different rifling twist rates at different times of manufacture and you should be able to go to Ruger's site and find out which twist your rifle left the factory with. Just send them the serial number and they can get back to you rather quickly. Depending on the bullets you are shooting, this alone may make a huge difference in your overall accuracy. You may want to try lighter or heavier bullets depending on twist rate, and see if this made the difference that you are hoping for.
Good luck either way.... I love the Mini!!! They ain't perfect, but they sure are neat little carbines!