And I reckon Leg, that at the time you and I got our hands on the M60 they were probably 20 years old at least. They had been well and truly used & abused.
I twice had a -60 go runaway on me due to wearing down of the notch on the operating rod that was supposed to engage the safety sear. The first experience was "What The Hell! OMGWTF!!!!!!!" - fortunately only had about 8-12 rounds left on the belt.
The second experience was "Oh, this again'. Had about 20-25 rounds on the belt and no Number Two so I decided to ride that one long burst just for the hell of it (rather than try to break such a short belt with one hand).
Both times happened in the early 1990s so those guns were pushing 30 years old by then.
I found out that later builds of the M60 (i.e. the M60E3 & E4) featured two or three notches on the operating rod, I think to prevent that particular problem.
Ah ha! Found an image of what I mean and I see that there were rods made with two notches and rods made with three. I have no idea if that correlates to the E3 and E4 versions respectively but I wouldn't be surprised as it can be seen on the image below, the two-notch rod has some wear on the first safety notch.
Seems to me that the extra notches are a decent way to avoid throwing out an entirely still usable operating rod plus they'd give a minor decrease in overall weight for a much better safety factor.
Image from here
http://beltfeds.com/m60-operating-rod-assembly/
I felt the same way as you Leg, 3-5 round bursts were my preferred choice but part of that was also to do with doctrine. When I was first being trained as a section gunner, we were taught the normal rate of fire was "bursts of 3-5 rounds, as required". I can't recall the specifics without digging out my old notebooks but I believe that fixed lines was the only rate of fire that required a burst of 10 or so rounds.
When I did my Sustained Fire Machine Gun course, the highest normal rate of fire we would employ was I think Double Rapid which was bursts of 10-20 rounds.
(Not to mention the anti-aircraft ROF which was taught officially as being the weapons cyclic rate and unofficially as "hold down the trigger until you shoot that bastard out of the sky")
As an aside, because I had the experience with the -60 and if I was feeling a bit cheeky (or perhaps smug is a better description!) when we had new guys being trained at the range, I'd deliberately squeeze off one round every second or so and smirk at their lack of ability to get a burst lower than 8-10 rounds.