I think it would have been much easier to balance the M14 for Commando. Just lower damage and reload time and it becomes great versus smaller zeds and weak versus large ones.
That's the problem: It's basic game design that fast firing weapons need to be weaker then slow ones to preserves balance. A full-auto will always be weaker (in a non-realistic game) then a semi-auto, which will be weaker then a bolt-action.
This is because the more quickly a weapon fires, the easier it is to target multiple enemies in a short time (crowd control). Hence, a quick-firing weapon by default leans more toward anti-crowd capability, while a slow-firing one leans toward anti-single spike damage.
Another property of a quickly firing weapon is that it can stack massive damage on a target extremely quickly quickly if your aim is true for each shot. This is why quick weapons cannot also be capable of spike damage with single shots (like a headshot bonus).
Hence, the addition of a quick-firing weapon (that also takes advantage of headshot bonuses) to the anti-single spike damage perk was illogical from the start, and figuring out how to balance it I'm sure has felt like fitting a square peg in a round hole for many of us.