-2 instead of -4 because a musket may be braced and more easily steadied, even when using it one-handed, unlike a sword, axe, whatever, that has to be moved (swung, thrust, etc.) without being able to brace the awkward weight. Through the course of my research, I also came across all kinds of interesting anecdotes and stories about instances of one-handed use that, along with my personal experience with such firearms, made me decide to go with the -2.