OK I found this... seems the differences are rather small... I did not know this... I've been slacking on my firearms knowledge...
The differences between the two are extremely small. The M4 has a 14.5 inch barrel, while the AR-15 generally has a 16 inch barrel, although multiple sizes are available.
The primary reason for this is the law in the United States about short-barreled rifles. This law requires that rifles have at least a 16 inch barrel.
As a result of this shorter barrel, the M4 has a shorter gas tube. To deal with the decreased pressure as a result of this shorter gas tube, the M4 also has more pronounced feed ramps to ensure that the next round is seated properly each time.
The barrel of an M4 has an hourglass shape cut into it, which allows for military members to mount a grenade launcher to it. The last big difference between the two is that the M4 has either a three round burst or fully automatic firing option.
https://www.dailyshooting.com/different-ar15-vs-m4/
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