Names and nomenclature do make a difference in military topics. You’d like to be as accurate as possible, and I think it’s important to use the terminology current at the time unless you specify you’re using something contemporary.
I’ll use two examples that have caused a lot of confusion—musketoon and the rifle-musket.
The difference between a musket and a rifle is simple—the rifle has barrel grooves to give the bullet a spin (and is much more accurate as a result), while musket barrels are smooth. A musketoon is just a shortened musket, usually meant for artillery or cavalry use. The US Army fielded such a weapon in the 1840s and 50s-the 1847 Springfield Musketoon, which was simply a shortened version of the 1842 Musket, a smooth-bored .69 caliber weapon that was at time the standard infantry arm. It was decidedly unpopular with the troops. If you’d like more details, there is a lengthy article here.
What about shorter rifles? Since they were rifled, they were generally referred to as short rifles or carbines. Some were just shortened versions of the standard long infantry arm, others were purpose made. The Enfield rifle, for example, came in three versions—the Pattern 1853 Rifle-Musket (39” barrel, three barrel bands), the Pattern 56, 58, 60 Short Rifle (there were several models with minor variations, all had a 33” barrel and two barrel bands), and the Pattern 61 Carbine (24” barrel, two barrel bands). British Army publications of the time refer to them this way, and there is no mention of the term “musketoon.” Here in the US during the Civil War the carbines usually went to the cavalry while the two-band short rifle was a favorite of Confederate sharpshooters. A small number of the older 1847 Musketoons saw use in the war, and some of those, like a few of the 1842 Muskets, received rifling later in their service life.
So where did the term Musketoon creep in to apply to a rifled carbine? Apparently with the venerable Parker Hale company, now out of business, who reportedly advertised their reproduction 24” Enfield carbine as such. You can find the term today on the present Davide Pedersoli web site to describe the shortest Enfield repro, but they call the similar Cook and Brother arm a carbine. Some authors have even applied the term musketoon to the Enfield Short Rifle.
Accuracy, then, would dictate the we call the shortest rifles (and we are talking about the ones with barrel lengths around 20-24 inches) carbines, and the intermediate length rifle (around 33 inches) a short rifle.
This leaves us with the rifle-musket, another term that causes confusion. Given the above definitions, how can one arm be both rifle and musket? And what about a rifled musket?
Infantry in the 1860s fought in a line of battle, which consisted of two ranks standing with the rear rank slightly offset. The rear rank men pointed their guns past the front rank men. Their guns had to project enough past the faces of the front rank men so as not to place the muzzle blast in their faces, which meant a barrel length of about forty inches or so. The US and British armies both used smooth-bore muskets during the first half of the 19th Century, but began to switch to rifled arms in the 1850s. They called the new guns rifle muskets in order to clarify that they were the same length as the muskets they replaced i.e. suitable for the line of battle. The Model 1842 Springfield Musket, the Springfield Model 1861/63 Rifle-Musket, and the Enfield P.53 Rifle-Musket all fall into this category.
What about a Rifled Musket? Isn’t that yet another contradiction in terms? Having accepted the superiority of rifled arms, the US Army retroactively rifled a number of the 1842 Muskets and the 1847 Musketoons. These .69 caliber weapons were still classified as muskets but since they had been rifled (the recoil was wicked) they were renamed rifled muskets to differentiate them from the unrifled versions.
All this terminology went away at the end of the war, along with the smoothbore musket. However, some vestiges remain. Standard army combat cartridges are still referred to as “ball” ammunition, even though the Army has not used a round ball since the 1860s, and a general court martial can still for certain crimes sentence you to be “shot to death with musketry.”