My nominee is anything designed by David Dardick, and more precisely, the "tround" guns.
I'd have to disagree with you. The basic "tround" concept is workable, but it's a case where the materials technology just hasn't caught up with the basic design. Couple the Tround ammunition handling and rotary chamber with a caseless cartridge, and the helical magazine concept pioneered by Calico, and you might have a decent stab at an alternative to the HK G11. I have heard rumors that the initial design by HK for the G11 was highly derivative of this idea, and the only reason they went with the clockwork nightmare they eventually settled on was that the Dardick patents were held by a large US defense company that wanted way too much money for them. That, and the same thing that drove the downfall of the Wankel engines--Sealing the two sides of the "breech".
Problem is, Dardick was way ahead of his time, and the materials technology and chemistry just isn't there to enable his ideas to work. Yet. Give it another twenty years, and we may see some of his ideas in another format.