'Musket' is a fairly broad term, but I think most people would take it to mean an 18th-century, smoothbore, flintlock, military long gun. The British Land Pattern ('Brown Bess') and French 1777 (which had many names and was widely copied) are probably what spring to most people's mind.

These muskets could be, and were, used to fire at targets below the shooter. Typically designed for use in an open field, and usually carried unloaded on the march, it wasn't usually relevant - but it did happen. Horatio Nelson was famously shot by a French 'sniper' using a musket from the rigging of the Redoubtable, which would have had to be aimed at a very steep angle (firing small arms like this was common practice - the British Royal Navy even experimented with specific weapons for it! Although it had started to fall out of practice by Trafalgar.)

This was made practical by a paper pad wrapped around and on top of the ball. Typical cartridges at the time consisted of a greased paper tube holding powder and ball - the musketeer would tear it with their teeth (often retaining the 'ball' half in their mouth!) and pour most of the powder into the gun, then add the ball and its end of the cartridge. The typical land use doctrine was to then ram the gun, and prime it using the remainder of the powder - but the 'powder' end of the cartridge could be added on top of the ball if you wanted extra sealing. The paper was heavily impregnated with animal grease (note: improvised cartridges weren't always, but French, Prussian, British, etc arsenal produced ones were) to aid in weatherproofing, which also helped them 'stick' the ball to the barrel.

I’m a Glengarry Light Infantry re-enactor. Generally speaking to the Brown Bess or carbines, Loaded muskets “can” be held upside down… The reasoning behind holding a musket upside down (loaded or unloaded) would likely be to keep the barrel dry to prevent rust, and the gunpowder dry as to not just flash the pan if it was raining. For live- firing muskets (I’m saying this because we don’t live fire in the field).

The process would be to bite the cartridge, pour enough powder in the pan to catch the spark and ignite through the touch hole. Next you would put the wad into your barrel and proceed to ram it with your ram rod. This does several things. As I’m guessing you’re talking about smooth bore rifles, without a tight packed wad your shot would likely be less accurate (to the point of accuracy these rifles can actually achieve), and having a tightly packed wad is essential so that the energy produced is used for the shot instead of the surrounding area if it were not packed tightly.

Saying this, we often carry our muskets upside-down (with the stock under the arm) to keep rain out. But if you haven’t rammed a shot, your powder will fall out of the barrel. I’m not sure if this has been specially written about, but it is still practiced in Napoleonic and Civil War re-enacting. So to directly answer your question, the wad should always be packed in well enough so when holding the musket barrel down the shot doesn’t fall out. There are accounts that the union army shouldn’t have been surprised in Shiloh because they heard musket fire, as removing a wet wad from the barrel of a musket is basically like drilling for oil. There are many sources that explain the detriment that rain had on Napoleonic/Civil War battles. (Honours BA history, in my MA soon specialization in military history… on my way to get verified)