Where does this 8" group stuff come from? Was this from a new or used gun? I have never owned or seen a Mini-14 that was that inaccurate unless it had been abused or had a sloopy stock fit. I have owned 3, 1 blued and 2 stainless ones. The blued one would shot 2" all day long with iron sights, if kept clean as any other gun. One stainless would do 21/2". The other stainless would only do 51/2" because some yo-yo that had it before me, never cleaned it and it must have had an eroded throat. Took hours to clean when I got it home. If you have a new or a good used one and it won't group. Examin the stock, it may need to be tightened up with some bedding work. Bed the heat shield into the stock from mag well to forend tip, which also can be bedded to fit tight into the gas block. Also bed the mag well frame/reciever guide?, into the stock. At last aply bedding to the top of the stock under the reciever and nearly but NOT completly lock the trigger asembly back into the reciever. When done right it WILL tighten up the stock fit and your groups. The only other expense might be a trigger job or barrel re-crowning.
I think this gun has a bad rep because it tends to have the heck shot out of it between cleanings. I've seen ones with the finish burned of the forends and wood handguards(on the older ones). Remember this is a military type firearm and about 3" groups are cosidered acceptable for these types of guns. I do agree about the Mini-30 wrong barrel diameter for the caliber unless you re-load better off with an sks or aks.