Member "Browning" has some good posts in this thread. Read those.
I'm one of those cops who didn't start out as a "gun-guy", but wanted to be and became a lot more knowledgeable during my LE career, but still learning.
MINI-14. To answer the OP, if there was a particular series of standardized tests, perhaps we'd figure out what "ruggedness" actually means.
Some thoughts on the Mini 14/30. I've only owned an older, first series (I think it was a series 181, unless there was even an older series). I was only a little bit concerned if it ever broke, which it never did, as it's my understanding Ruger would somehow give credit or a discounted price on a new one since the older series was not being supported with no old, dedicated parts available.
This was pre-internet and somewhere along the line someone must have mentioned to me it might break something, like an extractor, but again, it never broke while I owned it. I probably didn't put more than 1,000 rounds through it. Accuracy was fine for me at 50 yards.
MINI-30, no experience. I've seen a few at the range, but never owned or even shot one. I bought an M1A Scout and still have it. I must have a few thousand through the M1A, perhaps 3K-5K rounds.
AK47. Same 7.62x39 caliber as a Mini-30. I've got a (Yugo) Zastava "NPAP Mod. 70" through Century Arms (aka CAI). AK47 guns are many and can all be different. Some are unreliable new, out-of-the-box. Others, like mine (so far) are fine. I've only shot it twice, however, totaling 520 rounds, all steel-cased TULA ammo. I'll get to 2K rounds, at least, later this year as I've recently just bought it.
Do see this guy's Youtube channel (AK Operators' Union) for specific sample-of-one reviews on various brands and models over the recent years:
https://www.youtube.com/channel/UC0-nZ9dKQroaCr2MuAXeqyg
AR-15. I was issued a Colt AR-15 when I was working. My first one, new, out-of-the-box, and a problem destroying gas rings. I couldn't get through 5 rounds before there were failures to eject and failures to feed. I actually remember wishing I could just use my personal Mini-14, above. I was then issued a somewhat used department AR-15. This worked OK.
Some problems back then (~2001) was Russian steel-cased ammo (for practice only) would often fail to feed. Putting a rod down muzzle-end of the barrel would easily free up the round. I still see this occurring at the public range with many owners' AR-15s using a variety of ammo.
The other problem was the FIXED stock loosened up after rattling around in a patrol SUV for several thousand miles and lost some kind of spring-loaded detent parts. I'm not an AR-15 armorer and still don't know what this detent's purpose is for (edited to add: I believe it was a takedown pin detent & spring which tensions the takedown pin).
Otherwise, the AR-15 was generally reliable.
Final Thoughts. Any of these can (& will eventually) break. Any specific rifle may outlast another, despite what these samples-of-one do. To me, some other important considerations are: which rifle do you just want?; ergonomics; caliber; your intended use/mission; possible anticipated accessories, if any; anticipated target/threat distances; overall price (initial purchase, ammo); when/if it does break, how will it be get handled and how long will it take; if necessary, consider a spare to fill the "need".
