Wheeler has it right. I bought one of the first Mod 64 Cooey rifles offered for sale in Canada back in the day. At first I thought I had found the perfect gopher, raccoon, squirrel, grouse annihilator. All was well until that ridiculous plastic magazine started to wear, which was only after a brick of ammo. New mags were unobtainable and Cooey (Winchester) wouldn't honor their no fault guarantee. The company was in financial trouble and this policy didn't help. Later, some entrepreneur made up a replacement mag out of pot metal which wasn't any better than the plastic mags.
Cooey/Winchester dropped what was actually a very good design. The original rifles were very well made and finished. They even functioned well after a brick of rounds unless the mag started falling apart.
Well, along came another adaptation that we called the "Key mag." Why, because it loosely resembled a key. Again, the problem was that it was made from pot metal.
That crazy mag keeps plaguing what is actually a good design. Not only that but different corporations, like Savage, later picked up the patents or rented them, then jobbed the manufacture of parts out to different bidders. Quality control went to hell. Today we have a semi auto rimfire that with some fine tuning, like polishing contact surfaces etc.
Savage, for some unknown reason, stuck with crappy cast pot metal mags. They will give trouble.
I recently picked up one of these rifles with the intention of modifying or making up a new mag. Modifying an existing sheet metal mag is much easier. Mind you, Maybe something with feed lips similar to the Ruger 10-22 mags may just be doable.
IMHO the only real problem with all of the 64s, no matter who made them is the poor magazine system. As far as jamming goes, the vast majority can be blamed on that poor system. Another thing, the return springs in these rifles was designed to handle even the hottest ammo available and as such doesn't do well with standard velocity or inconsistent velocity ammo. The one I have now was greatly aided by removing a coil from that spring.