Complex sentence: Open handguns are often custom built with parts and features specifically designed for competition, and with the maximum magazine length of 170 mm some.38/9 mm magazines can hold up to 28 or 29 rounds.
Split into simpler sentences: Open handguns are often custom built with parts and features specifically designed for competition. With the maximum magazine length of 170 mm some 9 mm magazines can hold up to 28 or 29 rounds.