Per Wiki: "Deceleration bullet traps direct bullets into a helical or circular chamber in which the bullet may circle until it loses velocity and drops to the bottom of the chamber for collection. For use with multiple firing positions, the helical chamber often resembles a horizontal pipe, into which bullets are directed by upper and lower steel plates. The upper plate slopes downward and the lower plate slopes upward to a horizontal slot in the side of the helical chamber. Alternatively, some narrow deceleration traps for single firing positions employ vertical plates to direct bullets into a helical deceleration chamber resembling a vertical pipe from which spent bullets drop out the lower end. Lightweight versions suitable for capturing airgun pellets are often used behind electronic scoring systems on ranges configured for ISSF 10 metre airgun events. The plate slopes are shallow (about 18° ) so bullets may be deflected relatively intact rather than disintegrated upon impact. Some deceleration traps use an oil and/or water coating to reduce friction and capture dust."
Below is a photo of a P-51D using such a device. I noticed this photo this morning on a few facebook sites. Credit to Granger Davis.
Also per Wiki: "Below a Conceptual schematic cross-section of a deceleration bullet trap with black lines representing steel plates: Bullets entering from the left are deflected into the top of the deceleration chamber. Bullets are then deflected clockwise around the deceleration chamber until they lose kinetic energy and fall out the bottom of the deceleration chamber."
https://en.wikipedia.org/wiki/Bullet_tr ... 4oEC7VuCoA