Not all 250 grain bullets are created equal. A very stout jacket is needed to stand up the the .454's maximum pressure, stouter than many bullets have. If the bullet jackets aren't stout enough to handle the strain as the bullet hits the forcing cone they'll deform or fail with all kinds of bad side effects.
Since you're reading manuals, get ahold of the newest Speer manual if you don't have it. Their explanation of the reasoning behind choosing their 300 grain unicore JSP vs their 300 grain high pressure JHP applies to selection of 250 grain bullets as well.
Tom
