This might be a round that was designed to launch "bullet trap" rifle grenades. There were 3 types of rifle grenades. The first worked by using the energy of an underpowered fired bullet (as above) to launch the grenade off the end of the barrel. The 2nd type was made to be used with regular ball ammunition. The problem with some of them was that a full powered bullet would go right through the grenade and the warhead, causing a rather large explosion about 2 feet from the firer's head. Not a pretty picture. To avoid this scenario, many armies, including Canada's, where I picked up this tidbit, used the 3rd type, grenades that were fired using an overpowered blank round. The grenade firing procedure with the FN-C1 (FN-FAL) was:
-remove magazine
-clear any loaded bullet
-insert black tipped (or was it a red tip) blank (a real blank would not do the trick)
-reverse the gas plug so that no gas goes into the piston
-place grenade holder on barrel using the bayonet lug
-place grenade on holder
-pull pin
-place butt of rifle against ground and tilt the rifle towards the enemy
-tilt helmeted head so that helmet is between your head and the muzzle, and,
-pull trigger.
at this point there is supposed to be a big kaboom. If the improper round is used, there will be an even bigger kaboom.
The tracer element in the round pictured above would allow the firer to follow the trajectory of the grenade.
It is a possibility, and it does sound good.
Dean.