Is retraining soldiers on a different rifle complicated?
When reading about designs of rifles it's often stated they it operates like the old rifle to be familiar for soldiers. I guess an prime example of this is Russian AK series.
But is it actually hard to retrain soldiers? For example France switched from FAMAS to HK416, Germany G36 to HK416 and China Type 81>95>191. Did any of them had problems switching?
How about WW2 where lots of small arms where rushed into hands of troops? Did M1 carbine require much training or did troops just pick it up from the depot if available?