Huh?  The M4 was directly based on the M16, which was based on the AR-15, which was all ultimately based on the AR-10, which was first made in the mid 1950s, thus making it almost exactly as old as the B-52 bomber.<p>There&#x27;s a lot more differences between a B-52 bomber when it first rolled off the production line and what they&#x27;re like now than there are between an AR-10 and a modern M4, though of course the rifles are a lot less complicated.