WRT arrows, I remember reading some analyses of medieval battlefield records a long time ago, and I've heard it said many times by historians, but I can't point you to a definitive source.
WRT muskets, they are slow to load and fire, inaccurate, low-range, the bullets move slow enough that you can see and dodge them - good for massed warfare during the transition from cold weapons to hot, but not at all for the modern battlefield. IIRC it was in battles during the US Civil War that rifles which were accurate over long distances and allowed a small number of soldiers to defend a position came to the forefront.
Deadliness also doesn't seem to be the selection criteria for modern weaponry. If it were, militaries would all be using chemical weapons, firebombing, nuclear bombs, hollow-tips, poisoning water supplies, etc., but they aren't. Killing soldiers can be a part of strategy, but from what I see usually more of the strategy is about controlling key locations and resources, and winning through logistics and politics, while destruction is largely strategic or symbolic.
WRT muskets, they are slow to load and fire, inaccurate, low-range, the bullets move slow enough that you can see and dodge them - good for massed warfare during the transition from cold weapons to hot, but not at all for the modern battlefield. IIRC it was in battles during the US Civil War that rifles which were accurate over long distances and allowed a small number of soldiers to defend a position came to the forefront.
Deadliness also doesn't seem to be the selection criteria for modern weaponry. If it were, militaries would all be using chemical weapons, firebombing, nuclear bombs, hollow-tips, poisoning water supplies, etc., but they aren't. Killing soldiers can be a part of strategy, but from what I see usually more of the strategy is about controlling key locations and resources, and winning through logistics and politics, while destruction is largely strategic or symbolic.