LOL...hmm, Black powder SBR? That might be kinda cool....Then figure out how to have a can work on it!
I measured from the inside of the barrel from the back of the breech to the muzzle. 18" on the nose
And well over 26" overall.
You've got me thinking though....do the length regulations for cartridge rifles apply to black powder (since they technically aren't recognized as a firearm?) I would assume they still do.