My favorite is an AR 5.7 with 50 round mags combined with FN 5.7 pistol with 20 round mags. Same ammo for both guns!
The 5.7 is what I was too lazy to look up, but that is what I had in mind as the "perfect" set-up.
The AR-15 and it's pistols, 9mm pistols and short rifles/semi-firing submachine guns and so on are all the right tools, but for perfection in zombie guns, your package is the absolute best.
Here is someone's comment from a forum: ""(2) VERY low recoil, so you can see your hits if you’re shooting varmints out to 200 or so yards; (3) 50-round capacity; (4) Simple, blowback operating system; (5) Major versatility with all the rails to mount loads of accessories on the thing; (6) WAY less noise than a .223/5.56, in case you need to do some shooting in an area where excessive noise would be a problem; (7) Excellent accuracy;""