I have been loading 405gr .458" cast bullets backwards into 45-70 cases for years.
I started when I first got into experimenting/making silent loads in my 22" barreled T/C Contender carbine in 45-70.
A few grains of a fast handgun powder held in place with a filler, bullet seated backwards deeply into the case looks like a semi-wadcutter.
When fired the hammer hitting was louder than the shot, could get less than 2" groups @ 25 yards using iron sights and the bullet doing just under 300fps would go thru a 2X4.
The idea of seating the bullet backwards comes from looking at a water drop the fat heavy end is always down supposedly cuts thru air resistance better.
Sure smacks when the bullets hit...