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Comment by tempestn | Hacker News Reader

Do bullets actually fall at exactly the same speed if dropped or fired horizontally? In a vacuum, obviously they do. But my intuition says there would be some impact on the vertical air resistance from travelling 500m/s in the horizontal direction. As the bullet pushes through the air, the air pressure above and below it is increased (you can actually see that in the photo in the post). So wouldn't it effectively be falling through denser air than in the dropped case? Also, of course, it would be falling in a horizontal orientation, whereas the dropped bullet would probably realign itself to vertical.

Anyway, none of that invalidates the broader point, and the effects would be comparatively tiny even if present. Just found it interesting to consider.

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Fair warning, you are standing on the precipice of a very deep rabbit hole :-). My Dad was a 're-loader' (loaded his own ammunition) and molded his own bullets and had charts and graphs for an amazing set of things. I even built him a chronograph so that he could "know" the speed of a bullet as it left the gun.

Given that a 'zeroed in' sight will account for the drop and so points the barrel up slightly, some of the energy in a shot goes to lifting it "up" before it starts falling down. You might think that would make it take longer, but it's small. The sin of 0.5 degrees is only .00873 so less than 1% of "heading up" before heading down. The more 'up' you point it, the bigger the difference. But as the author points out, the impact of various factors varies. Dad was happy he could put three rounds from his 30-06 into a 3" target circle at 200 yds.

That's a bit of a mistaken model, the bullet never rises above the boreline of the gun, the scope or sights are just angled down relative to the bore line to intersect the bullet trajectory at a particular point further down range. [0] The effect is larger if your scope is higher above the bore axis, because it requires you to point down more on the sight line to have the same zero, or if your zero is particularly close or far, for most rifle rounds and gun configurations 100 yard zeros tend to be all drop (the combination of realistic height over bore and bullet trajectories just work out that way).

[0] This is a pretty accurate representation: https://s-media-cache-ak0.pinimg.com/originals/40/f3/05/40f3...

Another accurate representation, from the article:

https://militaryrealism.blog/wp-content/uploads/2025/03/figu...

If the sight is angled down relative to the barrel, the barrel is angled up relative to the sight. But which is angled "straight"?

The shooter aims with the sight, so if the target is level with the shooter's eye, the sights are level and the barrel is pointed (ever so slightly) up.

This means you are basically correct, in that the boreline is always pointed higher than where the bullet actually is (setting aside any lift the bullet generates, but that is a lot of detail to get into). However, the bullet does often move upwards relative to the ground (for some distance) since people are often aiming almost level to the ground. So in that sense, the trajectories are not "all drop". That is why I think you are being downvoted.

Yeah, should have said all drop relative to the point of aim/sight line I suppose, wrote for an audience more familiar with guns than HN probably is. Things are usually reference relative to the sight because that's what you're aiming with so movement relative to that is how you'd adjust your aim to actually hit the target.

There's not really any mechanism for lift though, the bullet is not an airfoil and is traveling directly into the airflow so there's not going to be any lifting body effects either while it's in stable flight and after that it's tumbling so there might be some then but it will be chaotic and inconsistent and you're just as likely to have it pushed down if the tail is up.

I think you misread ChuckMcM. Of course it’s the sight that is angled down. What he wrote was accurate.

Is there really a difference?

GP clarifies "the bullet never rises above the boreline of the gun" which tells me he might have thought that Chuck was saying the bullet literally goes up after coming out then comes down, instead of the boreline being pointed up relative to the pointed-down sightline.

I assumed that's why Chuck put "up" in quotes, because it's not actually going up, but seeming to with respect for the sight line.

> As the bullet pushes through the air, the air pressure above and below it is increased (you can actually see that in the photo in the post)

That's just a 2d slice/projection of the air density, the shock wave you see is actually conical and symmetric around the whole bullet it's just not possible to capture with schlieren photography.

As far as I know the pressure above and below are basically identical the bullets are symmetric and until they're well subsonic (the transition between super and sub sonic is a lot of what defines the effective accurate range of a round because the transition is chaotic so it's hard to predict and account for it's path after transitioning) they're traveling so fast forward there's effectively no angle of attack to generate a differential on either side of the bullet. If it does generate a negligible amount of lift it's completely dominated by the force of gravity pulling it down.

Yes, of course the shock wave is conical; I just highlighted above and below because they're the only directions likely to influence how quickly the bullet falls.

I don't think the effect would generate lift per se, assuming a horizontally fired bullet (since, again, it's symmetrical). What it does seem to me though is that it would effectively increase the density of air local to the bullet (in all directions except for behind), and so it would essentially be falling through denser air than it would be if just dropped from stationary. That would result in a small but non-zero increase in the time taken to fall the same distance.

Again though, that's just what intuition tells me; I certainly haven't tried to model it or anything.

The shock wave doesn't actually touch the bullet on the sides though so it doesn't provide meaningful lift. Look closely at the schlieren the high pressure region isn't against the sides of the bullet at all.

Air is already quite dense at sea level equivalent to ~194 dB sound pressure so the sonic boom pictured which is 130-160 dB is not a significant increase in the density/pressure of the air.

> exactly the same speed

Within a couple hundred yards within the capabilities of normal riflemen, basically yes. In real life though, there are tons of effects. E.g., there's a minor lift/drop in a crosswind because of the pressure differential from the bullet's rotation. E.g., when you have a gun properly sighted in and shoot at a horizontal target, the barrel will be tilted up slightly, the bullet will be tilted up slightly, the rotation will mostly preserve that upward orientation (like a gyroscope), and you'll have a lower descent rate than you might expect for roughly the same reason an airfoil does (and shooting upward or downward can amplify or have the opposite effect). Beyond a few hundred yards, all kinds of things matter.

The Mythbusters did it for real once. I'd call the results as "within error bars, consistent with same speed drop". Lift does not seem to be a significant factor. Shooters would have noticed. They notice a lot of things.

This is very interesting to consider! I think the point about orientation is a great one. Definitely if the drop was substantial, this would be an issue. Let's say you were standing atop a tall tower in a desert and did the same experiment, then maybe you'd see a slight but noticeable difference. The effects of air resistance would only become significant as the vertical velocities increase with acceleration due to gravity.

The aerodynamic effects you mention are real, but they are subtleties. Unfortunately, I have not been able to find a high quality, open-source ballistic calculator that actually breaks out all those details. Apparently, "Modern Exterior Ballistics: The Launch and Flight Dynamics of Symmetric Projectiles" is the book to read if you want to know more.

Although considerable basic research is funded by the DoD, and considerable open-source has been generated by them, and of course the DoD has done a tremendous amount of research on ballistics, I suppose it makes sense that they have not actively encouraged the publication of an open-source artillery targeting program.

There is an excellent MythBusters episode where they demonstrate a dropped and fired bullet hitting the ground simultaneously.

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