Good question. Given the bore to tube wall thickness, I (who have taken a chance or two
) would be pretty darn hesitant to fire that thing with a good charge of powder. Then again, I think that it would have been shot with shot rather than a single massive projectile.
OTOH, noisemakers typically have a little lip to hold a priming charge when they are upright.
What would you like it to be?
I think that it is well mounted and certainly could be used as a weapon. It certainly looks the part.
This really illustrates a core problem with our hobby.
In the years, say, from 1400 to 1600 how many weapons and elements of armor were produced? Think of all of the wars. Millions? Tens of millions?
OK. How many exist? 1% of the total made, less?
Therefore the argument for dismissing an object's authenticity is based on a sample of (presumed) real pieces that is in absolutely no way representative of anything other than itself.
That said, one has to be very careful in saying "I never saw one with (or without) thus and such".
I have no doubt that your piece is old, I have no doubt that it could be lethal. Though it's construction looks iffy to a man who shoots a modern S&W handgun, that is a relatively meaningless observation.
I think the correct answer to your question is that there is really no compelling evidence that it is anything other than what it purports to be.
BTW ... I'd gladly put it in my own collection, for what that is worth.