The ONTOS did have six 90MM recoiless rifles, they're manually loaded and don't have near the rate of fire that a battery of M2s would put out. When reloading, the crew is pretty well exposed to side and rear fire.
I suspect in both cases (Dusters/Ontos), both have been replaced by superior, proven weapons. I believe ONTOS was used extensively at Hue', Tet, 68, by Marines. Dusters were most assuredly used, with great effect, as described above.
Old fashioned soldier here. I am so damned old, when I was in the service we still used wooden rifles...and I still do. Absolutely LOVE M-14. Well, ok, M1A.