The barrel on the M60 is easy to change....
..it's one of the features that does work well. Clear the weapon, lock the bolt is to the rear, grab the release (in front of the receiver at the rear of the handguard) push it from left to right and flip the lever on the right side up. Using the asbestos mit, or grabbing the bipod legs if you don't have one, pull the barrel forward and set it aside. Grab the new barrel and insert it, then flip the lever down. The whole thing can be done in seconds. Of course if the AG installed the gas piston backwords in the other barrel, you've now got a 23 pound bolt action rifle instead of a machine gun. Just another design flaw. Weapons designed for use by GIs should be as fool proof as possible. It wouldn't have taken much, a keyway or some other modification to either the gas piston or the gas cylinder to make it impossible to put this in backwards, but it has been a common cause of malfuntions since the weapon entered service.