Good Morning Danny, thanks for asking.
YES your inline is a "muzzleloader" by legal definition in most if not all states. It is NOT a "traditional muzzleloader" by definition here. (Another interesting side topic..., all of what we classify here as "traditional" muzzleloading firearms are considered "antiques in most states, but that is based on their design being "copied" from an actual antique. I can't begin to tell you how many guys pull the 209 primer from their inlines in my state and drive home thinking they are in compliance with Maryland law, when Maryland Law does not -or didn't until recently- recognize an inline as an antique muzzleloader. So it was supposed to be fully unloaded when placed into the car and driven on the highway.)
The discussion cutoff date is 1865, not pre-1865, PLUS the ignition for the firearm must be from a an exterior source, not an internal primer from a cartridge. So certain breech loaders that load a cartridge BUT use an external cap, i.e. the [original] Sharps, The Burnside carbine, The Gallagher carbine, The Hall rifle, The Smith carbine, The Ferguson rifle, are all open for discussion, but the Spencer and the Henry, made for rimfire cartridges or modern cartridges today, are not, and neither are inline rifles.
We also don't discuss converted rifles like a Snider converted Enfield, etc.
The cap-n-ball revolvers are one of the minor exceptions to the overall rule. However, one may find SA Army 1873 revolvers, made with cap-n-ball cylinders to get around some European laws, and these, although cap-n-ball, are too modern of a design (1873)
You may discuss making of caps, and molding bullets, but may not discuss sabot ammunition nor jacketed bullets, nor may these be offered for sale in Classified ads, nor may the sale of black powder or caps be done (including "throwing them in to sweeten a sale").
OF course what you discuss with another member via PM is your business
Mostly this is for liability. The admin does not wish for the forum to become embroiled in some case where a prohibited adult person, or minor, got hold of a converted to modern use firearm, or blew themselves or others up, based on openly shared information gleaned from this site. There are plenty of other sites out there that have this information, let them deal with such risks.
LD