Some barrels are really skinny and might not support a really deep jug choke as some 'smiths' like to grind. However, the choke can improve patterns even if it is only a few thousandths of an inch deep. The reason is as the shot, then wad column enters the constriction at the end of the barrel, the shot column is squeezed tightly together and then the wad is retarded slightly, allowing the shot to maintain it's tightness as it starts flying freely, without the wads pushing into it from the muzzleblast.
I have almost an Improved Cylinder choke, about 010" on my 20 bore flinter and due to the black powder loading, it throws beautiful Modified patterns using standard card and fiber wads. I shoot an 82gr. measure of shot and a 75 gr. measure of 2f. .125" card on the powder, 1/2" fiber wad, then shot, then thin "B" wad over the shot. It also shoots nicely for closer range shooting with just the 1/8" card, then shot then "B" wad.