Well, no, the jam is not a cowboy action thing. It is very common in many of the 1894 rifles that are 5 years or older. Bottom line is, certain round nose .38 specials, and some round nose .38 +p, won't feed, because a second round gets caught under the carrier while the carrier tries to lift the first round into the receiver.
Because of this, Marlin invented an "improved carrier" that is now standard in all of the Marlin 1894s. They even call it that if you call them on the phone and ask them about a carrier. (They don't even sell the old carrier any more.)
If you have jams problems, call Marlin, arrange to send your rifle back for replacement of the carrier (total costs use to be around $75), and they will install the new one for you. THEY DO THIS ALOT! I have never heard of anyone with the improved carrier having the jam problems.
Best Regards,
Big Paulie