I'm with SMS on this one because it sounds like you have no idea what you're doing. 5.56 can be a far superior cartridge in every respect to 5.45x39. And it can excel in many aspects compared to 7.62x51. FMJ's have never been excellent at killing anything. If you want to excel at killing anything, get the right projectile, or lower your standards on what you're actually capable of doing.
If you really desire to find whats right for you, you need to seek to own and then sell each of the guns that fit your bill and make a self educated guess. I'd roll with some form of AK-47 with an FSC47 comp on the end, and an Aimpoint on the top rail.
Just to keep the conversation flowing...one needs to define "Tactical", "SHTF", "Reliable", and "Accurate".
Tactical: needs to be defined...I cringe when I see it, think it's over used.
SHTF: in what context? A single deadly force encounter? A brief period of unrest, or a long term collapse?
Reliable: How many failures per xx round count are you willing to accept? Under what conditions? Reliability is subjective.
Accurate: Sub MOA, 1+ MOA? At what range? Custom ammo, walmart ammo, milsurp ammo?
Each answer can lead you towards a different requirement...see what I'm getting at? Additionally I hope you see why it'd be hard to say "buy this" or "you need this" given the nebulous criteria we've laid out so far.
I went a little different way for my SHTF, Bugout rifles..AK for crowd control, ground cover..WASR 10
SAA/DD M-4 built by a Fine gunmaker (may not be the fanciest ,but it has shot every time) couple decent ..cheap scopes for each...both ..under $1000.
I buy bulk ammo from ..ammoman, ammotogo, cabelas...its all going down range.
But then again Ive had good luck with.....Taurus pistols...Good Luck