The barrel is 3.5"!
Whatever rounds you choose to carry need to be ran through the gun at the range just to make sure that it will feed the properly. I carry Critical Defense rounds in my M&P pro9 and have put about 50 rounds of them through the gun with no problems. I will be trying them in my 9c the next time that I go to the range. Until then it will just get fed FMJ's.