@Jason SamsonGoogle "Ghost Guns" a recent "Tizzy" about how some individuals have crafted or manufactured firearms in the USA for more than 300 years.
You may craft / manufacture a firearm for your own use (as long as it is not fully automatic aka a Machinegun) and the ATF does not care. A registration number/serial number is only required if you transfer (sell) that weapon to someone else. Magazine capacity may be regulated at the state level.
A "One Hundred Caliber" projectile would be approximately 25.4mm (a Fifty Caliber projectile is 12.7mm; M2HB .50 HMG is 12.7x99mm; however a "Fifty Caliber Pistol" fires a much "stubbyier" projectile, perhaps 12.7x30mm; compare to the typical 9x19mm pistol ammo).
Consider that a .454 Casul pistol or .50 AE "Auto-Mag" are both "Wrist Breakers" and not "concealable" so you are basically crafting an abomination suitable for the Guiness Book of World Records, but not a practical weapon.
Another example is the M82A1 Barret "Light Fifty" AMR (Anti-Material Rifle) in most configurations weighs around 30 pounds (14kg). The M242 Bushmaster Chaingun used in the M2A2 Bradly Fighting Vehicle and several other platforms weighs more than 300 pounds...
The Israeli Mossad is suspected to have assassinate the individual who was manufacturing the "Worlds Largest Gun"; he was a Canadian, developing a gun (cannon) for Saddam Hussein in the late 1980's. The GUN was based upon several "Cross Channel Guns" build by the Nazis c.1941/3 in the Pas de Calais to shell England. It was believed that Saddam Hussein intended to fire chemical projectiles at (in the general direction of) Israel {across Jordan}.
Perhaps Putin would be in the Market? Or, maybe the Hegemon of Uzbekistan? Maybe Mongolia... Ah-Ha! The Dictator of North Korea!
lots of people getting worried about who he's planning to point it at, but the state seemingly unconcerned.
"Celebrities" such as the cackling hens on "The View" or Justin Bieber and other "Has Been" stars might stir some shit. Nobody who actually understands firearms would be concerned.
If it is a "One Hundred Caliber" pistol it is utterly impactable, possibly un-firable unless the firer is wearing Power Armor such as in Heinlein's Starship Troopers. If it is a "Longarm" (aka rifle) it would have to be the size of a small cannon... if it is a Cannon (auto-cannon, Howitzers, or Guns) well, 16" Guns (406mm) Naval Rifles aboard the Iowa class Battleships (and other warships), or even the 18" Guns on the Yamato class Battleships would seem to indicate "You Need To Up Those Numbers!" 100 Calibre? "Those are Rookie Numbers Son!"