That drawing comes from the older Testors/Italeri M4A1 76 turret. Other drawings include the M4A1 "Bonnie" a marking variant in the Italeri kit. The kit gun tube was incorrect with a step. I built this kit as a kid and learned about the gun tube years later.

This website may help:

http://www.usarmymodels.com/MANUFACT...taleri225.html