When using a stethoscope for taking blood pressure you're supposed to use the Bell Side of the Stethoscope which is the smaller portion of the stethoscope.
However I've seen some doctors use the Diaphragm portion which the larger part of the Stethoscope when taking blood pressure.
But I've always been told to use the bell portion of the stethoscope when taking blood pressure for better results.
The Bell of the Stethoscope is used to detect low frequency sounds and the diaphragm of the Stethoscope is used to detect high frequency sounds.