The Ruger Mini-14 was originally created back when the 5.56 NATO cartridge was first coming on-scene with the AR and M-16 platforms. People were generally socialized to the M-14 pattern of rifle, so Ruger kinda sorta scaled down the M-14 (I know, there are important mechanical differences between the platforms. But from a user perspective the similarities are compelling).
I humbly suggest the purpose of a Ruger Mini-14 is approximately the same as the AR-15. It fires the same bullets at essentially the same muzzle velocities. It accommodates magazines of arbitrary capacity. It's semi-automatic. The primary differences are the "furniture" (the Mini-14 being wooden stocked) and the manual of arms (the platforms differ on magazine release, safety actuation, etc. Not a big deal, but they exist).
The Ruger Mini-30 is essentially a Ruger Mini-14 chambered in 7.62 x 39. The same cartridge used by the AK-47 and (most) variants.
Edited to add: I ran the ACOG equipped Mini-14 through a 4 day practical rifle class out in Nevada years ago. It served well, providing accuracy that was commensurate with my talent at the time. On the night shoot before the last day of class I could do no wrong. I was nailing my targets decently fast. The next morning I couldn't hit the broad side of a barn. Nothing wrong with the rifle or optic. I just had a great night and a less-great morning (and no, I wasn't hung over...

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