Most large game rounds have a fairly long throat. Eg in the 375 H&H and 9.3x62 the throat is approx 3/8" long. In the 458 Win mag its around 1.1" long. Most Weatherby cals have throats that are 3/8" long, whereas they used to have 3/4" long throats back in the 80's. When it originally came out the 458 Lott used to have a long throat like the 458 Win mag but it was later shortened quite a lot.
In some cases it's for chambering long, full diameter RN pills and in other cases it's to allow a smaller case perform like a larger case by quickly dissipating pressure (eg 458 Win mag).
As you have found, in your case it may not be practical to seat the Barnes bullet as recommended for accuracy.
