Considering very low BC, and very light projectile for a full sized (.620") 'bullet' - the loss of only 250fps seems a stretch.

According to Speer, a BC of .160 and if started at 2,300fps MV., is running 1,807fps at 100 yards.
That's a loss of 493fps.

The .58 cal. Lyman #575494 bullet is 315gr. and has a BC of .067. Started at the highest vel. Lyman shows, of 1,300fps,
that one loses 416fps in the first 100yards.

An even lighter bullet of larger calibre, started even faster will have an even lower BC & more drop.
Speer's data shows for a BC of .120(their lowest), a loss of 640fps in the first 100yards travel from a 2,300fps initial vel.

So, I am calling BS on his projected loss of 250fps for a 275gr. .20 bore bullet started at 2,250fps.

Good article, though.