Just some back of the envelope physics - Gravity would only slow the bullet slightly. Wind resistance makes more of an impact, but the bullet is still potentially traveling over 2000 ft/s if the shot was relatively direct (2500 feet straight up).

Specs

Fastest .50 BMG: 647 [grain] or 42 [grams], 3044 [ft/s] (928 [m/s]) out of a 45 [inch] (1.1 [m]) barrel. Acceleration due to gravity: -9.81 [m/s/s] 2500 [feet] = 762 [m]

Gravity

Position given original position, initial velocity, time, and acceleration (so we don't need to do calculus right now).

Assuming it goes straight up at the plane, which would give the highest possible velocity and energy.

x = x0 + v0 * t + 0.5 * a * t^2 x = 762[m] x0 = 0 v0 = 928 [m/s] a = -9.81 [m/s/s] (solve for t)

(Use Wolfram Alpha to solve the polynomial formula: 762 [meters] = 928 [meters per second] * t + 0.5 * -9.81 [meters per second per second] * t * t) (text input: 762 = 928 * t + 0.5 * -9.81 * Power[t,2]) t = 0.824716.

Velocity given initial velocity, acceleration, and time: v = v0 + a * t v = 928 [m/s] + -9.81 [m/s/s] * 0.824716 The bullet is still going 919 m/s without wind resistance at 2500 feet up assuming there's no air, but it goes fast, so there is quite a lot of air.

Wind resistance

Hornady estimates a .50 BMG that starts at 2950 [ft/s] will be going 2085 [ft/s] at 2500 [ft]. (https://riflebarrels.com/barrel-lengths-velocities-for-the-50-bmg/) This would be at standard temperature and pressure. In real life, there would be less wind resistance as the wind resistance decresases with altitude due to lower pressure.

It goes about 70% of the original speed assuming constant pressure.

Since gravity only slowed the bullet an inconsequential amount, we can work with a number that has a little bit less energy than this (round down for ease of math).

We can reasonably determine the bullet to be traveling more than 2000 [ft/s], or about 600 [meters/s].

Collision energy

E = 0.5 * m * v * v m = 0.042 [kg] v = 600 [meters / second] E = 7,560 [joules], or 5500 [ft*lbs].

That's still quite a lot of energy, and an ideal shot could have been more than that.

For comparison, per a quick Wikipedia check, a 45-70 at the muzzle puts out 3,449 [ftlbs] (4676 [J]). A 30-06 puts out 3036 [ftlbs] (4116 [J]).

The .50 BMG put more energy out at 2500 feet in the air, and makes a bigger hole. It would likely damage any aircraft. It may not immediately make the airframe unairworthy, but definitely damage would occur, especially if multiple shots hit.