"... I discovered why Henry's shotgun cannot be
accurate with the commercial rifled slug-the slug did not
fit any part of his choke-bored barrel; the cylinder portion
measured .731" and the muzzle .705", while the rifled slug
averaged .685". This made it a loose .046" in the cylinder
and .020" in the choke. Thus, the slug was certainly safe
to shoot even in a tighter choke, but never accurately,
hecause each shot would strike the taper of the choke
constriction at a different point, resulting in a deflection
from the line of aim with practically every shot. The claim
that the slug expands to the diameter of the barrel doesn't
seem to hold water."
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That is why George Vitt set out to design a better shotgun slug back in the late 1950s. His slug design stayed on the handloaders market from the early 1960s until the 1980s.
US manufacturers finally increased the diameter and thus accuracy of common "rifled slugs" in the eighth decade of the last century. Wow that was some 40 years ago!
Oh yeah, here is an updated
Classic Guns link to the article on page 29 of the December 1963 issue of
Guns Magazine:
https://gunsmagazine.com/wp-content/uploads/2018/12/G1263.pdf
