Questions of firearms identification, including bullet identification and determining the flight path of bullets, are recognized as areas where qualified persons having certain expertise, rather than lay witneses, may testify. A common fallacy in the area of firearms evidence is the labeling of a witness involved in firearms identification as a ballistics expert. It is essential for attorneys to qualify a witness in the right context to avoid having a witness excluded from testifying. Another concern in qualifying a witness is whether or not the testimony to be given is based on principles that have gained a minimum level of scientific acceptability. A firearms examiner may express an opinion based on the experiments of others, the knowledge from journals or academic treatises, or the opinion of other witnesses possessing expertise in the particular field in question. The first source of information under the Federal Rules of Evidence is the firsthand observation by the witness present at trial. In such a case, the witness may answer hypothetical questions or criticize the opinions of other witnessess at trial. Although this leads to 'a battle of experts,' an attorney can expose weaknesses in the method, approach, and conclusions reached by an opposing firearms examiner having another firearms examiner analyze the testimony of the expert called as a witness by the opposing side. The third source of the expert opinion may be based on the data presented to the expert outside of the court. However, a party has the right to cross-examine the witness and question the use of the particular data on which the opinion is based. In a criminal case, counsel representing the opposing party has the right to cross-examine the person responsible for any underlying data used by the witness. Therefore, an attorney should be aware of the requirements of qualifying the witness possessing particular expertise and should recognize the sources used by the expert witness. An attorney should be aware of such factors as the expert's professional and educational background, reputation, and court experience. The firearms examiner should assist legal counsel but should never be permitted to control the presentation of the evidence. Fourteen references are included. For related articles, see NCJ 75559-60 and 75562.