Not entirely sure, Basically it's not completely Sphereical.
If placed in the barrel with the seam at 90 Degrees to the direction of fire (if that makes any sence) it fits, how ever, if it enters at any other angle its too big. Basically, the seam is the right size but either side has swollen. At first i assumed it was just the first bag as it had, had a break in transit. But with closer inspection, it's the whole lot!