Hi Gus,
That is a good question. Many late 16th and early 17th century guns had tangs tapering to a rounded or squared-off point. It seems that a slightly loose fit of the tang to the wood was common. To be sure, 400 years of wood shrinkage and wear and tear are partly or perhaps the entire reasons but I wonder if they didn't leave a little space intentionally. Of course the fit of the breech certainly helps to share and distribute the force of recoil. Later, in the 17th century tangs were most often parallel sided or flared. I wonder if that didn't happen because of recoil concerns as opposed to simply fashion. The way I deal with the problem when I build a gun is I scribe the sides of the tang with a razor sharp carving knife for a close fit but mark the end of the tang with a sharp pencil. I then cut to the pencil line, which automatically provides a slightly looser fit. I also bevel the sides and end of the tang.
dave