In my XCR, there is quite a difference between shooting the older corrosive ammo (the stuff in the blue paper boxes) and the newer stuff (in the plastic bag).
The newer stuff is about 200 fps faster than the old stuff. It also tends to jam up in the chamber after about 20 rounds or so. My feeling is that because it is hotter, it expands the case just a little more than the older, slower stuff, and since the XCR chamber is likely a little tighter than the SKS or CZ chambers, once it warms up the lacquer on the case causes it to jam.