Propper answer and to clear a few points up:
On the whole they both use proprietary mags anyway - P/Emags and the AKM mags, the ACR being able to use SOME M4/16 mags, but they can have feed issues using non-magpul mags due to fast cycling.
AKM will not even hold an AK mag, they simply don't fit. feed hole in the wrong place.
AKM mags fit into M4 pouches at the same scale of 1xM4=1xAKM
AKM has a shorter barrel and folding stock.
AKM has larger mags (160rds over 120rds)
There are no hop up issues in either gun, only A&K models have them.
Range/accuracy is very similar between the two models. Only with propper measuring in a controlled environment could anyone genuinely tell the difference between them.
AKM is more expensive (by £50) due to the barrel difference (new barrels are £110) and a different (better) stock (retails for £80) - do the math.
Out of the box absolutely nothing has to be done to it, some people insist on putting a tightbore in them, but all that does is ruin the airseal from the factory for no noticeable gain in performance.
AKM does have different motor and trigger setup (some fine tuning I believe) and appears to - extensive test firing of both models - give a sharper trigger response, but neither rifle is sluggish.
If you play CQB get the AKM.
If you play woodland, get whichever you prefer the looks of.
