I'm pretty sure it's model also. I vaguely remember hearing the word "model" in some sort of US military documentation, but I can't remember for sure.
If you look at the M16's history, you'll notice that when the US Airforce first looked to purchase that rifle, it was designated the XM16E1 (most likely "eXperimental Model 16 Experiment 1"). After the official adoption, the rifle was simply called the M16. It would appear that the US military just dropped the extra "experiment" letters from the acronym and simply called it the Model 16 rifle.