Well, OK. I'm not going to discount anyone's experience, but I'm still skeptical. I can't see how something that handles 30A easily will undergo a catastrophic failure at 60. The wire loop, probably #14 or larger, has a fusing current between 150A and 200A, so there is no way that 60A will melt it. It might melt the solder in the connections, but that won't be sudden, if it happens at all. Remember, the pointer is coupled to that loop only by a weak magnetic field, and it carries no current. Doubling it is unlikely to cause mechanical damage.
I suspect people who report ammeter damage have underestimated the short-circuit output current of the alternator, which could be a lot higher than the rated output. Or simply made a mistake in connecting it. Either of which is likely to cause damage, with or without a shunt.
As for shunts...you're dealing with hundredths or thousandths of an ohm resistance, most of which is in the connections, not the wire. A minuscule change in the connections (like adding an extra washer) can make a proportionately huge change in the resistance, which will strongly affect the full-scale reading. You can't be sure the shunted meter is correct unless you check the calibration with the shunt in place. That's one reason why I've never used one, as I don't have the equipment to calibrate it.
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