Where the hell do you get that? You change the barrel when the weapon starts to display misfires or a bread in the cycle of functions. Or generaly after about 10 minutes of sustained fire or 2 minutes of rapid fire.
The M60E3 was the last version of the M60. It was modified to make it lighter. Troop acceptance was extremely poor- it's lightening features made barrel change necessary after 100 rounds, not 200 rounds, and made it very unreliable.
Out of the 'big three' Soviet PK, the US M60 and the Belgian FN-MAG, the M60 is the worst. That's why the US Army replaced it with the FN-MAG (M240).
This is from a US Army FM on the PK:
Pros:
The PK is more powerful than the M60 (7.62x54R over 7.62x51mm)
Easier to handle when firing
2kg lighter than the M60
Cons:
The PK has 100m less range (1,000m instead of 1,100m)
The ammunition belt doesn't disintegrate
The Soviet PK has since been modifed to PKM standard (1971)- lighter but with the same performance, and the Petcheneg (late 90s)- with improved barrel/greater accuracy etc (the barrel is not changeable during combat though- no longer seen as neccesary)
The M60 has been in service in various forms with the US military since 1950, and has proven markedly inferior to its foreign counterparts, the German MG3 (a modernized MG-42 in 7.62x51mm), Soviet PK, and Belgian FN-MAG.
The M60 is a gas-operated, belt-fed, medium or ‘general purpose’ machine gun. It features a quick-change barrel, integral folding bipod and provisions for it to be mounted on a tripod or vehicle mount.
The M60s mediocrity is surprising considering its lineage: the first prototype was created from welding the belt-feed mechanism of an MG-42 onto an FG-42 automatic rifle (designed for use by the Third Reich's paratroops, the Fallschirmjager).
The M60 is known as ‘the pig’ not only for its distinctive grunting sound but also for its excessive weight and the difficulty of maintaining it. It also has a tendency to jam.
The M60 was always characterized as ‘barely adequate’: the bipod and gas chamber are permanently attached to the barrel, so quick replacement of the hot barrel in battle conditions was problematic. The shooter must wear asbestos gloves. Another drawback was the fragility of many parts of the operating group: key among these, and inherent in the design, was the firing pin, which seemed almost guaranteed to break right behind the forward shoulder.
The M60E3 was an attempt to improve the M60 by making it lighter and making several design changes, but it is a very flawed model and acceptance by the troops was very poor. The ‘weight-saving’ measures dramatically decreased the reliability of the weapon. The new, light barrel was capable of only firing 100 rounds before having to be changed, instead of 200 as on previous models. The reliability of the M60E3 was said to be even worse than the original M60, and this was one of the reasons for its replacement by the FN-MAG (type classified as M240).
