Sexton v. State, 93 S.W.3d 96 (Tex. Crim. App. 2002)

“The potential rate of error factor weighs in favor of exclusion. Crumley testified that this technique is one hundred percent accurate. Vincent DiMaio, author of one of the treatises on which Crumley relied, testified that he could not be one hundred percent accurate in matching two cartridge cases based on their magazine markings unless he had the actual magazine. The Court of Appeals found it significant that DiMaio’s interest in firearms identification was only a hobby. DiMaio, the Court of Appeals noted, did not qualify as a firearm and toolmark expert,5 therefore his opinion should not discount the reliability of Crumley’s assertion that the technique employed was one hundred percent accurate.
Crumley’s bare assertion is one that the available literature contradicts, however. The literature says that these marks may enable an examiner to connect cartridge cases with the same weapon. The only literature that explains what circumstances make it possible for an examiner to do so requires that the examiner possess knowledge of the manufacturing process of the tool surface and have the tool available for creating test toolmarks. In this case, the magazine or magazines that made the marks upon which Crumley based his identification were not found by the police. Therefore Crumley was not able to make test marks for comparison. Also, Crumley did not say whether he was familiar with the manufacturing process of the magazine or magazines that he said left identifiable marks on the live rounds and cartridge cases.”