Quick question. Is it legal to chop down a muzzle loader to less than 16 inches or does that need a tax stamp?

its not a firearm

heck you can buy SBS MLs from Cabelas

ETA: Correction as long as its a muzzle loader that can accept rifle barrel (that would require 4473 paperwork) then I believe its considered in need of a NFA stamp

No stamp for muzzleloaders.

20 gauge smoothbore pistols, shipped right to your door, no stamp required.  

(g) Antique firearm (Title 26 United States Code)

The term "antique firearm" means any
firearm not designed or redesigned for using rim fire or conventional
center fire ignition with fixed ammunition and manufactured in or before
1898 (including any matchlock, flintlock, percussion cap, or similar
type of ignition system or replica thereof, whether actually
manufactured before or after the year 1898) and also any firearm using
fixed ammunition manufactured in or before 1898, for which ammunition is
no longer manufactured in the United States and is not readily
available in the ordinary channels of commercial trade.



       
Your state laws may vary, but muzzleloaders are not federally required to appear on a 4473, no matter if the barrel has rifling or is smooth bore.



Some confusion comes from firearms that are designed or modified to fire fixed ammunition, that also have capability to use a muzzleloading barrel or caplock cylinder. These have been "designed or redesigned for using rim fire or conventional
center fire ignition with fixed ammunition" and no longer fall under the protection of being an "antique firearm".



For example: a caplock New Army 1858 can have a shoulder stock attached legally without a tax stamp, but with a centerfire conversion cylinder would no longer be an antique and would require registration as a SBR.



 

I am referring to a TC Contender or CVA Apex where it is considered a firearm but does have muzzle loader barrels and thus unsure on it. None the less those are considered firearms with or with out and barrel.