After some research, 3/0 fishing weights weigh approximately 0.5g, while 1/4 slingshot ammo weighs 1.0g. So what you bought from McMaster seems to be the exact ones companies sell as slingshot ammo. Sorry for my lack of research before replying the first time, your darts will not be allowed with those weights.
Just weighed some 3/0 lead fishing weights and they are 1g too. Not sure if what you found was the weight of another size of fishing weight, but the 3/0 fishing weights I own (from Walmart) weigh the same as the 1/4" steel ball. I could use 3/0 weights in my darts, but I don't like to handle all that lead plus the steel balls are cheaper.