Jon,
It sounds like what you made was essentially a set of 'super tubes' like Browning used to sell. I think they were 16", not sure if that was because of the BTAF rules or because of some internal ballistic reason, but would suspect the former. One of the interesting things about short tubes is that the choke in the bbl's end is the one that does the constricting .. that applies to short chamber reducers, and short tubes both.
I suspect there is probably some magic number for the minimal distance for that to take effect for each sub gauge in a 12 bore or said another way, if there is insufficeint space from a tube's choke to the muzzle then the tube's choke will dominate.
Does anyone know the numbers? Probably a good subject for an article.
