How often do radiologists miss the mass and get other tissue during an ultrasound guided biopsy for a small (7mm) breast mass? Is this common?

No. The diagnostic accuracy of ultrasound guided breast biopsy is 94.7%. Here is a reference: http://www.Ncbi.Nlm.Nih.Gov/pubmed/21215163.
Not common. As a pathologist looks at these biopsies and i can tell you that the radiologist are very good very careful about making sure they have biopsied the lesion it is very rare that the area was not on the biopsy under ultrasound and when it is not the radiologist immediately contact me to you understand this discrepancy this is rare when the ultrasound lesion was not adequately sampled.
Not often. Ultrasound guided biopsy is very accurate, at least 95%. And although 7mm sounds very small in real terms, in the world of ultrasound guided biopsy it is a decent sized target.