Complex sentence: In small arms, a "long gun" is generally designed to be fired braced against the shoulder, in contrast to a handgun, but may include mounted firearms while in artillery a "long gun" would be contrasted with a howitzer or carronade.
Split into simpler sentences: In small arms, a "long gun" is generally designed to be fired braced against the shoulder, in contrast to a handgun. In the context of cannons and mounted firearms an artillery "long gun" would be contrasted with a howitzer or carronade.