The penny is not a good reference point. Would be better to measure it just below that crimping cannelure with a caliper and weigh on a scale that weighs in grains versus grams. Off hand on a guess, and this is a guess only, I would say something like a .32 caliber (as in .32 long), 98 grain lead round nose.
By your comparison appearance it almost looks small enough to be a .25 cal., like a .25-20 but they were generally a jacketed flat point and .25 acp used no crimping cannelure as they used a taper crimp. Dunno hard to positively id like this.
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