I found the following statement in a late 30’s Western Ammunition Handbook. I’m not sure how they proved this??
A Bullet Dropped and One Fired From Rifle
Hit the Ground at the Same Instant
It is hard to believe, but true. Two bullets, one fired from a rifle placed exactly parallel to the ground, and one dropped from a point exactly the same distance above the ground as the rifle barrel, will strike the ground at the same instant. The pull of gravity on both bullets is exactly the same. Of course, the bullets must be of the same size, shape and weight---there should be no wind, or practically none---and the ground at the tow points where the bullets hit would have to be at exactly the same level.