We hold that the trial court properly considered the knowledge, experience and training of Christensen prior to qualifying him as an expert, and we will not disturb the sound discretion of the trial judge . . . Although Christensen had no formal degree in ballistics, he did testify on direct examination that he trained under a Dr. James Booker, the director of the crime laboratory and the firearms examiner at the Wyoming State Crime Laboratory. Further, he studied books on the subject of firearms identification and had done hundreds of comparisons for class characteristics on projectiles. Christensen has testified on seventeen occasions in courts of Wyoming as a firearm's examiner. . . Through employment, Christensen demonstrated his expertise in the field of firearms and ballistics. He had been trained by an expert to make bullet comparisons. He had special knowledge that could assist the jury, and the trial judge found him competent. . . Plus, an expert witness is not limited to direct examination but exposes himself to cross-examination to thoroughly test his expertise.