I saw nearly the same comment posted somewhere and have looked everywhere to find it again, I can't recall who said it but it gave me the impression they had used 10s of thousands of the KVB-556M successfully and I wondered what the difference was.
AFAICT,
KVB223 - normal small rifle primer, non magnum
KVB223M - same charge as above, thicker cup (e.g. for autoloader/AR use)
KVB556M - magnum charge and thicker cup, more like CCI #41
not sure this is correct i asked powder valley about this before i bought mine and this is there response
KVB223 are not suitable in an AR. An AR firing pin could puncture the primer cup. If you like and use Federal Small Rifle primers in any application you will love the KVB223 Both of the M primers are suitable in the AR. Opinions are endless regarding which one is preferred. It really comes down to personal preference. The 556M and 223M are hotter than the 223. The 223M is reported to be the hottest.