According to Rear Admiral William A. Schoech, assistant chief of the Bureau of Aeronautics for Research and Development, the muzzle velocity of the shells was 3,000-feet a second. Their speed through the air (the muzzle velocity plus the airplane's speed) was about 4,300 feet- per-second,
"This would be more than 2,000 miles an hour, but their speed was immediately slowed down, because of air resistance. The plane was traveling about 880 miles an hour, better than 100 miles an hour faster than the speed of sound."
If the airplane had kept its original course, it would have passed by them, but its steepened dive path made it intersect the bullet's down-curving path. When it hit them, they must have been moving so slowly that the airplane overtook them at a good fraction of its own air speed, which was about as fast as many a newly fired bullet.