is semi-auto essential, useful or irrelevant for a survival situation?
Everyone uses the military as the justification for semi-auto large cap magazine rifles for survival. This to my mind is irrational for two reasons: 1) Logistics. Truth of the matter is, there are now 12 support personnel to every 1 trigger puller in the USMC (there were only 8 when I first joined in the 80s and I did a hitch in the arms room). The Army's tooth to tail ratio is even higher, like double. Part of the support system is larger arms rooms, with access to supplies of parts (firing pins, springs, magazine springs & followers, gas tubes, etc). After 2 tours with an Infantry battalion (forward deployed, daily fighting in hostile climate - hot, dusty, etc., with user maintenance) I am convinced that the logistical support requirements for a gas operated magazine fed platform are larger than I want to manage, much less saddle my wife and kids with if I'm killed or injured. Granted I've only carried every TO infantry weapon issued to Marines since the mid-80's 'til now, so I can't speak to the maintenance and reliability of other platforms, although my guess is that it would be the same or less than what we carry, not better.
2) Likelihood of force on force firefights. The military exists for force-on-force engagements. Large capacity rapid fire weapons are a tactical advantage, but not by themselves. There's combined arms doctrine, fire and maneuver, and whole lot of other things that are more vital to success than sheer volume of fire. If you're planning to need large cap rapid fire weapons for survival then perhaps "survival" isn't what you're interested in. Maybe "revolution" or "resistance" is really what you're preparing for, in which case a semi-auto weapons would be preferred, as well as grouping together with other folks, all of whom use the same platform with the same ammo, and maybe somebody who's been trained to coordinate fire drills. Your little group will need an armorer for each dozen shooters, with parts and training, as well as a steady supply of ammo. If you're on mixed platforms, you just exponentially complicated your job.
If the weapon's primary purpose is survival with a secondary use of defense or offense against superior numbers, then I would not go with a semi.