Hi Daryl

It's my understanding that the term 'rifled musket' was simply a transitional name applied to that group of military firearms that entered service in the 1850s and 1860s as replacements for the older smooth bore muskets. These weapons were typically a mix of purpose built rifles and re-barreled 'muskets'. In the case of the latter, the re-barreling was done as a standard exercise in military cost-cutting. Not unsurprisingly, the firearms of this transitional era were/are often described as rifled-muskets, although, as you point out, a rifle is a rifle and a musket is, in reality, a smooth bore.

Not too sure about the grain weight of the projectiles used in the 1853 pattern rifles. I haven't shot one of these in over 25 years but I'm fairly sure we used 450 grain projectiles (among other things) in our Short Enfields back in the day, however, I couldn't tell you now if they were dropped from a traditional or modernized .577 mould.