The annular confinement beam or ACB was a vital component in virtually all 23rd century to 24th century transporter mechanisms. These safeguarded the matter stream of an object, while the quantum factor was separately beamed to the target. The ACB generally used back-up from the energizing coils and pattern buffer to ensure a successful and safe transport. It also had a discriminating filter for any potentially dangerous substances. Disruptions in the cycle of re-materialization yielded a large release of energy with risk to an increase of signal degradation. Sometimes attributed to a "Pauli Effect", when the amount of matter was not compensatable to the given energy output. (TOS novel: Ex Machina)
This may explain why, even in mid-transport, Kirk and the others in TOS episode: "Bread and Circuses" beamed safely out of the cell, while bullets passed harmlessly through them. And such when disruptor fire did the same in TOS movie: Star Trek VI: The Undiscovered Country. The weapon discharge-deactivation from TNG episodes: "Datalore", "The Most Toys" was partly attributed to the annular confinement beam.