If it was a true Ruger Tactical and not a home made job of an older model it has a 1-9 twist. http://www.ruger.com/products/mini14...le/models.html
For a 1-9 twist bullets from 52gr up to 69gr should be fine. The 55gr is by far the most common. That said, I think you need to look at the rifle before you look at the ammo. The Mini has gone through a few changes over the years. Ruger has retooled their manufacturing and beefed up the barrel on the latest models. Although your 5 inch groups are well within Rugers accuracy specs of 3 inches at 50 yards, I think your mini could do better.
First the scope. What kind of scope and is it mounted properly? The Mini is notorious for beating up even the best of scopes. Not from recoil, but from the action and the ejecting brass. Scopes for the mini should be mounted turned 90* counter-clockwise. This puts the windage adjustment knob on top of the scope and out of the way of the ejecting brass. Check your windage knob for brass and signs of damage from ejecting brass. Then turn it counter-clockwise 90*.
Next the trigger. The trigger on the Mini is about the worst available. It has creep, over travel and about 7-12lbs of pull. Although a trigger job will not fix the rifle it will make you a better shooter. Send it off to Bill Springfield for the best Mini trigger job available.
Then the stock. The mini is no different than any other rifle were stock to barreled action is concerned. To get the most out of your Mini the stock should be bedded. It's simple to do yourself or you can have it done for you.
Last, the gas block. Ruger spits these rifles out as fast as they can. They mount the gas block wherever it lands all cockeyed and crooked then tighten the gas block screws to a couple million pounds of torque. Not at all good for accuracy. It's a simple fix. Remove the gas block screws with a good quality Allen wrench. These screws are staked so it best to work them back and forth until the stake is broken. Then align the gas block nice and even and set the gaps between the two halves equal with a feeler gauge. Then tighten the screws to just about 36 inch pounds. That's "inch" pounds.
All these tips and more can be found over at the Perfect Union. Follow this link and these guys will get your mini back on target.