Cams vs Knobs for BDC

[GeezerHood]

Are there any scopes out there that actually use a cam to change from one bullet drop compensation to another or are they all knobs that have the various ranges engraved on them for a specific round?

I know some people call them "cams" but I have never seen a rifle scope with a cam type adjustment system on it for elevation or for windage. They were all standard screw thread adjustments and you swapped the knobs to change the bullet drop scale as desired. One example, the Leupold M3 -- often mistakenly called BDC "cams" this scope has no cam type adjustment system of any kind on it.

Have any of you ever seen a cam type elevation system on a riflescope?

[sinister]

Leatherwood Automatic Ranging Telescope on an M21.

The eccentric cam (see knob) raised the rear of the scope to compensate for range as the shooter raised magnification. It is a very fast system but has some weaknesses.

It is an obsolete US military item. The design suffered from the spring often losing its ability to bring the rear of the scope back down as you cammed from far back to near. If you got grit, sand, or dirt between the cam and cam surface on the base you got varying elevation.

http://i221.photobucket.com/albums/dd233/heatseeker64/MT1_5592.jpg