 Eighteen were 40 caliber Mark IV weapons, thirteen were 45 caliber, and four were 50 caliber; apparently the 45 and 50 caliber guns were non-standard export models.These and the nine 6-inch Armstrong guns acquired at the same time appear to have been purchased to rapidly arm coast defense batteries with modern quick-firing medium-caliber weapons due to the outbreak of the Spanish–American War.It was feared that the Spanish fleet might bombard US East Coast ports.