If you notice both the M-4 and the AR-15, there is one common difference. AR-15 Vs. M4 is one hot debate to look out for.The M-4 is a fully automatic weapon that is available for only the armed forces. When you compare both the AR-15 and the M-4, you can notice that the barrel size alone differs. Many people are seemingly interested in the AR-15 Vs M-4 war.The AR-15 is known to use a 16-inch barrel. But when you see the M-4, it contains a 14.5-inch barrel.