1. Grab a MX 3GL 6.5 mm rifle and a few 40mm 3Rnd HE grenades.
2. Fire the grenades but notice how the rifle bolt animates as well as if you had just fired a shot.
3. Reload the GL, look at the side of the weapon and notice how the magazine of the rifle comes out and then goes back in, as if the weapon is going through its reload animation as well.

I could not reproduce this with the 1 shot 40mm grenades, nor could i reproduce this on any other weapon, but then again i couldn't load the 40mm 3Rnd HE grenades on any other weapon, either they are not compatible with these grenades or it is a separate bug entirely.