In P.O.'s testing, he used a .30/30 m94.
The rifle was re-chambered to the .30/30 AckIMP & the whole test was to show that the straight sided case would grip
the chamber walls prevent thrust against the breech-block after the lug was removed. The contention was that with minumal
cast taper, the back thrust was greatly reduced. I do not know the 'taper' of the .375 nor the .405, offhand, without looking it
up. Ackley's "taper" was usually .014" to .015" total, or 7 1/2 thou per side.