Comparison of slug type and speed for accuracy.
Comparing better 12ga slugs for rifled and smooth.

The US-S 570gr slug, with
locked on base does 2-3" groups at 50 yds
and about 9" at 100yds in heavy barrel.
At 2000 fps in smooth bore.

Now a Dixie 600gr slug in rifled barrel at 1300,
2-3" at 50yds- about 9" at 100yds;
BUT Dixie at 2000 2-3" at 50 yds and 6" at 100 yds.
It seems that slower slugs in rifled barrels losing velocity
gets below the speed of sound at 100 yds which
affects accuracy. Of course super heavy bullets, like
12-1500gr, make it harder to start out fast
enough to be supsonic at longer ranges.

This accounts for the guys info I posted earlier about his
high speed in rifled 20ga killing deer at 160 yds.
And holding tighter groups at longer ranges.
He kept velocity above the speed of sound
further down range......Ed