I don't think they were “undetectable” as such but I remember a type of revolver which had a cylinder with grooves instead of round chambers. It was fed from a magazine which contained "trounds". The trounds were plastic (glass reinforced Nylon) and held a bullet, power and primer. The shape of the tround was basically triangular but the tip to tip dimension was used to define a radius on what would have been flats with a much smaller blending radius on each tip. The result was a shape that behaved like a cylinder when feeding through the magazine and into the cylinder. If you dropped a few trounds on a table top and placed a book on top of them, you could move the book back and forth like it was on rollers.
The outside of the cylinder was contained in a thin cylindrical shroud which held the trounds in place. The trigger was double action and after a shot was fired, the cylinder indexed to bring in a fresh tround as the expended casing fell from a longitudinal slot.
I believe it was offered as a .38 and had interchangeable barrels of .22Lr and other calibers, it was a most unusual looking weapon which may have been its downfall. I also remember that you could separate the action from the barrel and mount it to a carbine although that might fall afoul of BATF rules.
Regards,
GtG
The furor over “undetectable” anything is just more smoke and mirrors, crap to keep the dummies exercised.