.458 Win Mag Shotshell Loads

[MedRiver]

Excellent info. Thanks. A few of those in my scabbard would likely keep the troops eating grouse!

 

[Riflecrank]

Simple as this, crammed full of shot and an inverted gas check firmly crimped over the top,
and a thin card wad between shot and 18.5 grains of Blue Dot, and any LR or LRM primer:

 

[Riflecrank]

I hardly expect to improve on simple for the purpose.
I did try another trick, but never shot them, still sitting here waiting for a pattern board at 10 yards:

Get rid of the cap on the shot capsule and you can get just over 500 grains of #7-1/2 shot with just a little protrusion of the delicate plastic shot capsule.
Even more weight and better patterning with #8 shot, just to perseverate on that point.

Now the round ball in a sabot awaits, to duplicate Davy Crockett's first rifle, Betsy, a .40-cal shooting a .395-caliber ball.

An inverted gas check could also be used to seal the sabot-ball load at mouth of case.
If it works out, I'll need a "Gemmer .458 WIN MAG" for BP loads, with a ramrod and a blow tube for wiping the barrel between shots at deer and squirrels.
The two-capsule shot shell load is a joke.
Hahaha.