30 Carbine
I am not sure which book you are quoting from, but there is little or no difference in bullet weights.
Also, we know the round was made in Britain. We have the examples, the specification and I have copies of the production drawings.
Using Imperial measure, which the round was originally specified in, the spec. for the US M1 Ball round has a bullet weight of 111 grns. The British bullet weight spec. was 112 grns and I believe the Hirtengberger bullet weight was 110 grns. Thus the bullet weight of both is within 1 grain of the spec. of the US round. That is far closer than manufacturing tolerances, so it is pointless to say one bullet is the heaviest.
This is military ammunition, not match ammo. Weights between individual bullets can easily vary by 4 grains (if spec'd, at + or - 2 grains).
Regards
TonyE
Source for US data - Frankford Arsenal Ammunition Specification Sheets.