There a reloaders that form their own brass from existing cartridges. A somewhat standard example of this is taking spent .223 Remington or .556 NATO brass, cutting it a little bit shorter, and re-forming it to .300 Blackout. This can be done at home, without setting up a whole new ammunition production facility.
For your fictional cartridge, you could start with a case from a larger rifle round, such as a .416 Barrett, or even .50BMG, and form it into your own fictional load. Look up ".950 JDJ" if you want to see something that Hellboy might shoot. It is a shortened .50BMG case with a solid brass 95 caliber bullet.
As far as NFA laws go, there are both "gotchyas" and "loopholes," in addition to how the laws are commonly interpreted. The Taurus Judge fires .410 shotgun shells, but it isn't an AOW or SBS, because it has a rifled barrel and is really chambered in .45LC, but with long chambers. The Mossberg Shockwave and Remington TAC-14 are not SBS or AOWs, even though they both have 14" smoothbore 12 gauge barrels, but instead fall into a somewhat nebulous "firearm" category, because they have an overall length greater than 26" and don't have a stock. The .950 JDJ isn't a "destructive device," simply due to the fact that it was granted a "sporting" exemption by the ATF.
Even with that in mind, legally owning an NFA firearm isn't as hard to do as some make it seem. It is just a matter of submitting extra forms, a $200 check, and an indefinite waiting period for an examiner to finally get to your paperwork (might be three months, might be three years).