Reyes is always referred to as "officer," and in fact wears the badge of an officer of the NYPD, but wears sergeant stripes on her shirt sleeves. If she were a sergeant, she would have been addressed as such, and her badge would have been gold and had an eagle on the top, which the standard officer badge does not.
As previously and correctly pointed out, "Officer" Reyes wears Sergeant stripes on her sleeves and a Police Officer's badge. The badges (known as shields) worn by Sergeants also correctly pointed out previously are topped with an eagle. They are also very obviously gold in color rather than silver color that Officers wear.
The lighting behind Watson fluctuates while she is talking with her therapist.
Holmes says that he can tell that the sub-machine-gun fired at Detective Bell had a rifled barrel rather than smooth-bore. All guns have rifled barrels.
The reason it is possible to tell that a bullet was fired from a rifled barrel from the bullet holes is because bullets fired from guns without it tend to hit the target sideways, with much less force and no accuracy.
Sherlock describes the weapon that fired at Detective Bell as a "semi-automatic", but then goes on to say it fired in 3-round bursts. A "semi-automatic" firearm only fires a single round per pull of the trigger.