I've been around a few decades and have experienced a few things, so let me add a tad to this discussion.
I've been chasing bambi with foster slugs in smoothbores since before rifled s.g. barrels were on the market.
The BRI was specifically designed for RIFLED, not smooth bore barrels.
The BRI will tumble immediately upon exiting the muzzle.
Also the BRI was designed to tumble when forward velocity decelerates to a certain point, usually around 150 yards.
The rotational velocity is unaffected by distance traveled and is of no consequence.
I was rummaging through my ammo stash and lo and behold I found 9 five packs of Federal 2 3/4" BRI s.g. bullets.
Several years ago I had an 11-87, with one of the 1st rifled, cantalever, barrels.
Using the Federal BRI load it would shoot consistant 2" 100 yard groups.
That s.g. became my "deer rifle" and accounted for 3 deer the 1st and only season I used it. The longest shot was 150 paces.
Performance was stellar.
The 'bullet' is already .50 diameter and has a big flat meplat to transfere all that energy to the target.
In short. A good load.
The 11-87 went away in a trade long ago.
I got into muzzle loaders and have had a great time taking a few deer.
However. Three seasons ago I watched a big brusier buck at 200 yards, almost every day of season knowing he was perfectly safe from Ohio hunters armed with revolvers and slug shooting s.g.s
I began to seach for a solid 200 yard performer.
I ain't found it yet in a s.g.
That said, with deer season fast approaching, and discovering my stash of Fed. BRI (all from the same lot number) I loaded up the 870 Express with Remington aftermarket 23" cantalever barrel with T/C 2 X 5 scope.
I shot 6 rounds into an even inch c. to c. at 75 yards, two inches high of bull.
That ought to put me in the K zone out to about 140 yards.
Much further than that the BRI has slowed enough to tumble and fall quickly.
Like it was desinged to do.
I have yet to find any sabot bullet in 12 bore that is consistantly accurate at 100 yards, let alone 200 yards. Despite the fancifull hype generated by the ammo companies or the odd report of the 'expert' shooter who occasionally gets three shots touching a gnats behind at 50 yards.
The 200 yard slug ammo does not yet exist in 12 bore.
Perhaps in the fanciful imaginations of some hunters, but not in documented reality.
That includes the fearsome 880 grain full bore bullets I've been playing with.
Shoot your BRI with confidence. They are very good sabots, but don't fool yourself into thinking you can kill deer at 200 yards with them.