(Long-Winded Diatribe Alert)
Of course I don't have the complete dimensions of the Federal Fiddlestick, but other than rim thickness, it looks like you could ream out that projectile ledge and you'd just about have a .50-110 Sharps rifle.
18 U.S.C. § 921's definitions don't seem to either fully include nor exclude this system as an "antique or antique replica," which is one of several filters it would have to pass in order to be a non-4473 arm - so it looks like ATFE is playing it safe and calling it a Firearm. But I remember that around 1998 they made the same classification on Knight's DISC system, so Traditions might have to sue and appeal (and appeal and appeal) if they want to peddle these things at retailers that don't have the proper FFL class and data & security systems to stock "firearms" and/or do Insta-Checks. If I recall correctly, in that case Knight and supporters successfully got language into an appropriations bill that amended the definition of "antique firearm" to include Knight's DISC system from the previous language of the 1968 GCA - and that happened under the Clinton Administration after the 1994 AWB and subsequent Republican majority transitions in both Houses. Thus, NitroFire might actually have had better luck under the Obama Administration's DOJ and BATFE leadership, who signed off on bump stocks and AR pistol "wrist braces" as Good To Go without NFA issues.
On that note - the deeranddeerhunting.com article states that the NitroFire can be purchased "Anywhere Traditions Firearms muzzleloading rifles are sold." They're a little bit ahead of themselves. There were a few blued Buckstalker and Tracker blister packs at my local Farm & Fleet store yesterday when I was out buying more sidewalk salt, but they will not be able to stock or sell/transfer the NitroFire as it is currently classified.
I guess I have mixed feelings. On the one hand, I enjoy living in this golden age of firearms and shooting sports equipment innovations, and wouldn't want to step on anybody's freedom. Since it's a "Form 4473" gun, I don't anticipate enough of these things hitting the woods to make Iowa's muzzleloading season woods any more crowded. Our Iowa Early ML season limits to 7,500 tags statewide anyway, and the late season tends to be at the time of year during which we usually see Arctic Blasts arrive - so those using "lazy people's" muzzleloading systems are either disinclined to endure the weather, or are at least paying their dues through other forms of suffering not related to the convenience of use built into their guns. And I do like the safety aspect of being able to extract both primer and powder charge for transport.
But on the other hand, as the deeranddeerhunting.com article states, "Hunting white-tailed deer during muzzleloader-only season is typically a way to spend more time in the field for a rifle or shotgun hunter. However, antique firearms and even modern inline muzzleloaders do have their frustrations and limitations. So much so, that the hassle of operating them can be a deterrent for some hunters and can be considered a barrier to enter the sport." Therefore any system that eliminates the "hassle of operating" deterrent, seems likely to invite more lazy hunters into the woods - during seasons specifically set aside for those who are willing to accept such "hassles" in return for a chance to hunt (at least in Iowa) without .450 Bushmaster ARs and autoloading slug-barreled shotguns going off from every third tree in the county, like our gun seasons have become on public land. Were it not for Iowa's ML seasons being limited as they are, I'm sure I would have given up deer hunting entirely by now since I'm not connected to any landowners who haven't already leased out hunting rights beyond the reach of my disposable income. Thus, I'm cautious not to cheer too loudly for new ideas until I'm somewhat certain it won't quickly push hunting one more step back toward being "the sport of kings" rather than the pastime of any interested, law-abiding, tax-paying Everyman. And I think in the case of NitroFire and Federal Premium Fiddlesticks, that remains to be seen.
I have spoken.