Originally Posted by Buffalobob
But we agree gravity is a downward force and it only affects things in one direction - downward.
(the effects of air drag will cause the actually added or subtracted velocity to be somewhat less than calculated).
This is true irregardless of whether you shoot in a vacuum or not.
I calculated lots of these types of examples in college 30-35 years ago in an engineering curriculum, but we always assumed a vacuum because that simplifying factor was required to allow the straightforward determination of a "correct" answer. I agree gravity is an ever present downward source of acceleration. It's ultimately the primary force that keeps a bullet that's been fired straight up from leaving earth's atmosphere. And it keeps a bullet moving downward until it's stopped by impact with the earth (or when we do everything correctly - a game animal).
But the atmosphere is present and it's a huge (overwhelming) factor controlling bullet velocity (deceleration) and ballistics (predictive modelling of the bullet in flight), making your estimate on the relative addition to, or subtraction from, velocity in the example a complete guesstimate. Gravity's affect on the horizontal velocity component (horizontal deceleration) of a bullet fired on a horizontal plane is truly negligible during exact horizontal fight, because the bullet is travelling perpendicular to the forces of gravity. And for angled shots somewhat above or below true horizontal to the face of the earth, the effect on the horizontal component of a bullet's velocity still borders on the negligible at high velocity in comparison to atmospheric friction, which is why it's not a necessary input factor in the computer models like Exbal.
I'm not a ballistician. Otherwise I'd be able to explain this more clearly. And even they won't try to explain the theory to an average layman. Do I think gravity plays any role in bullet flight in upward versus downward angled shots with high velocity bullets? Yes. Do I believe that role is significant enough to be worthy of consideration and incorporation in a predictive ballistics software program. No, because its affect borders on the negligible. It might serve a purpose if modelling the fight of an arrow or BB in predicting the long range angled shot ballistics of those slow speed objects.