M777 was the best gun at this time for Afghanistan. It has range and it can be airlifted.
The barrel life is I believe is rated at 20,000 rounds at top charge. A temperature sensor is mounted on the barrel, it has three ranges, "green", "yellow", and "red". You don't need to be a gunner to understand what the colors mean.
I have to check the gun manual but other factors can determind ROT(rounds on target). Weather conditions being one.
Dirt or damaged bands on the shell can throw off your calculations. Damp charge bags. Gunners job ain't easy with this new gun but its the best that is available considering the situation at the time.