There are a lot of variations on gun screws, You tend to find that mass produced guns have a slightly wider parallel sided slot with parallel sides to the nail heads which fit to a flat bottomed countersunk hole in the corresponding metalwork. For such screws you need a turnscrew with entirely parallel blade and edges to fit the screw. Those such as offered by Grace USA will fit many of the modern produced guns. Because of the difficulty in grinding the sort of tips on those grace USA turns