The AR-15 has a longer barrel, and is semi-automatic. The M4 is a carbine (short barrel) and is capable of fully automatic (burst) firing. The M4 cannot be owned by a private citizen in the US- they were made after the May 1986 cut off date to register full auto firearms.
The AR-15 is the family of rifles from which the M16, M4, C7, C8, etc. are derived. As for a military M4 vs. a civilian legal AR-15, there are several. AR-15 is a trademarked name which belongs to Colt - thus, only AR-15 rifles manufactured by Colt or by companies licensed by Colt can actually call their rifles AR-15 - companies such as the Cerberus companies (Bushmaster, DPMS, Remington) cannot call their product AR-15.
As for the differences between the two, the M4 has selective fire (either SAFE - SEMI - BURST or SAFE - SEMI - AUTO) vs. the semi-auto only of the AR-15. The bolt carriers are constructed different on the select fire M16 and M4 vs. the semi auto only AR-15s, as are the hammers (the semi auto only models use a smooth hammer). The receivers are milled in a manner so that a full auto trigger group or auto sear cannot be fitted into it as is.
The M4 is capable of automatic fire, the AR 15 is not
AR-15 = Semi-Auto civilian rifle M4 = Select Fire Military rifle
AR 15 - semi-automatic only M16 - depending on variant capable of full auto or burst fire. Will also fire semi-auto.
No.
M4 is the carbine- shorter barrel
The US Army presently uses the M4 carbine, a shorter and much modified version of the Armalite AR15.
AR15, M16, M4, Ruger Mini 14
Law Enforcement
Both are assembly line weapons. Stag in my opinion also has a better reputation. Unless you decide to go tactical, they are all pretty much the same. Bushmaster, dpms, stag ext..
Their is no NATO M4. NATO countries that use the M4 either contract the purchase or license the production.
There are a large number of differences based on the models of both guns, however the most notable is that the AR15 is a semi-automatic and the M16 is capable of semi-automatic or full automatic.
The M4 is basically a carbine version of the M16A2 rifle, and was developed both to reflect the development of the M16A2 over the previous M16A1, and to correct deficiencies which had been identified with the XM-177 (which was the carbine variant of the M16A1).General differences between the M4 and M16 include:M4 has a four position collapsible buttstock (vs. the fixed buttstock of the M16)The weight of the buffer is different in the M4 than the M16The M4 has shorter handguardsThe M4 has a shorter gas systemThe M4 has extended feed ramps cut into the upper receiver for ease of chambering rounds.The M4 has a 14.5 inch barrel, vs. the 20 inch barrel of the M16The M4 has an "F" marked front sight base, which is designed to compensate for the difference in sight radius between the M4 and M16The M4 has an indentation in the barrel profile to facilitate the mounting of an M203 grenade launcher.Some people often cite the removable carry handle of the M4 as one difference, but the first production run of M4 carbines had the same fixed carry handle as the M16A2 (I was actually issued one when I was in the FL National Guard).