I've never shot the -21, but I don't see how the ammo can would cause feed issues.
The -21 was designed to fire from an open bolt, unlike the -16. We had feed issues all the time with the -240, which also fired from an open bolt. They made several mods to the feed system in the -21, including one that grabs the rounds from both ends. Turns out it's just hard to put that big of a round into the chamber at those kinds of speeds. The -240 had a variable gas port that would change the rate of fire...hint, it hardly ever worked in the highest setting. If it did it would quickly get gummed up and have to be swapped. I'm unaware of such a system on the -21, which would lead me to believe that's where the problem lies.