If you ask me, the M4 has got a lot of things the MP5 doesn't. The .223 can go through armour if needed, but can also have hollow points or other types of ammo so when you shoot the bad guy, the round doesn't put a hole in everything behind him

. It also has a lot of aftermarket products (stocks, scopes, barrels, RIS, etc), it's lighter than most carbines with comparable barrel length and is proven. I think the M4 is a little long for CQB, but you can get shorter barrel AR type weapons. It's also worth noting that the AR-15 can be attapted to many rounds, from .22 LR to .50 BMG. This means less training between different platforms

. The MP5 is a good weapon, but for about $1000, compared to a little more (normally) for decent AR-15, it's a little costly, espically if your department is low on money (then again, if we went on price, we'd all just have AKs

).
On the HK 416, I think its a bad idea putting a different gas system into a rifle (ie, from Direct Inpingment to Short Stroke), but if it works, it works I guess
