The cardboard charger's are not as flimsy as they appear.
A while back we gave it a try and did fire & movement drills using SKS's and K31's. It worked like a charm. While we had to slow down out rate of fire as compared to using modern assault rifles, we were still almost quite as effective, but out fire was for sure more accurate.
But putting about 150 rds of GP11 through a K31 sure is quite a workout.