64.798 grains = 1 milligram.
Doing the conversion gives you:
115 grains = 7.45 grams or approximately 7.4 grams
124 grains = 8.034 grams or approximately 8 grams
148 grains = 9.59 grams or approximately 9.5 grams
158 grains =10.238 grams or approximately 10.24 grams
I am reasonably sure that these odd bullet weight numbers were standardized in accordance with european metric conventions, and then converted to the US avoidupois units of measure.
You will find similar weight conventions on all european calibers, where bullet weight is expressed in grams.
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