Rumor, Myth and Reality:
The world of the smoothbore seems to be immersed in all three!
Just what good are those shallow grooves on the lead thimbles we call Rifled Slugs?
After some searching, I located this 1991 DOJ/FBI Report which documents slow rotation as a contributing factor to rifled slug accuracy:
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"The slight rotation imparted by the ribs reduces the effect of manufacturing irregularities. In tests performed by Winchester-Western, the slug rotation was confirmed, resulting in consistently smaller groups for rifled slugs than unrifled slugs…
The basic Foster slug, however, remains a
hollow lead cup, heavier at the point. It has 14
small angled ribs swaged into the side of the
slug. The rifling tends to be obliterated by the
passage of the slug through the barrel, especially
through a full choke. Some spin does result
from the ribs, however, and tests show a very
slow spin of approximately one turn in 24 feet
of travel to one turn in 129 feet of travel,
depending upon the choke used. In 1980,
Remington and other slug manufacturers increased
the weight of 12-gauge rifle slugs to a
full 1 oz.
Neither Brenneke nor Foster slugs depend
upon the rifling ribs or projectile spin for stability.
The slugs are stable because they travel
through the air like a sand-filled sock with the
heavier toe forward (O'Connor 1965), unlike
symmetrical lead balls (Figure 3). The trailing
light end acts as a stabilizer. The slight rotation
imparted by the ribs reduces the effect of manufacturing
irregularities. In tests performed
by Winchester-Western, the slug rotation was
confirmed, resulting in consistently smaller
groups for rifled slugs than unrifled slugs
(Sterett 1966)."
Crime Laboratory, Digest April 1991 - Volume 18, No.2
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Full report here:
https://www.ncjrs.gov/pdffiles1/Digitization/132229NCJRS.pdf
What I find most interesting is the varying rate of spin noted for different amounts of choke constriction.