Depends on which military. US uses the M16A1 rifle, or the M4 carbine version of the same.
The military M4 is made by Colt's Manufacturing Company
The M4 is named after its manufacturer, Colt's Manufacturing Company, which developed and produced the firearm for the United States military. "M4" stands for "Model 4" as it is a variation of the original M16 rifle. The M4 is a versatile and widely used firearm by military and law enforcement agencies worldwide.
A limited number are in use with select units, but it will not be replacing the M4 and M16 in line units.
AR-15 = Semi-Auto civilian rifle M4 = Select Fire Military rifle
A semi-automatic M4 derivative can cost as low as $800 from ImpactGuns. But for a civilian to acquire a fully automatic, military grade M4, be ready to spend up to $3000.
The standard 5.56 mm cartridge is centerfire. There IS an adaptor to permit the use of .22 LR rimfire ammo for training.
The military gets a reduced rate for buying in bulk. IF a transferrable M4 exisited, it would be in the 20K range.
When they feel it necessary to do so.
The M4 is a military rifle capable of semi automatic or 3 shot automatic fire. It is used where a lighter shorter rifle is useful.
The M4 is a shorter version of the M16 rifle, which is the military designation of the Armalite Rifle. Eugene Stoner is generally credited with the creation of the Armalite.
Their is no NATO M4. NATO countries that use the M4 either contract the purchase or license the production.