[FrankenMauser]

Alright, I know the conventional wisdom is that an air gap in BP loads is a pipe bomb death sentence.

But is it really?

I have recently been loading homemade BP in my .577/450 Martini Henry, which was very easy with which to eliminate air gaps, since my amateur methods resulted in rather low density BP (but fast burn rate).

I have been reading reports all over the place, from books, to forums, to discord posts, to the distant tendrils of the interwebs.
Nothing agrees.

The only commonality is:
-People that think air gap is bad have never tried it. Or, they're referencing old reports that were small granulation (3f or finer) with a notable air gap to the projectile. More often than not, the "failed" firearm is a handgun or small bore rifle.
-People regularly shooting air gap loads in cartridges with no ill effect, but almost exclusively large capacity, large bore rifles, using 1F or 1.5F BP.

It is too late (literally, I need to go to bed) for me to go digging for references, but there were supposedly 19th century standard .577 Snider and .577/450 loads that had air gaps.

Where does this "air gap danger" come from?
And is it real?