HntnFshI really have not got a good answer or an alternative sabot for you to try. I have my 50 Mountaineer in the process of cleaning so I did try some alternative sabots with a 458-300 Bloodline and they were harder to load than the a regular red or orange MMP.
As I think I have said there really is not a lot of options with the 458 bullet in that there just are not optional sabots out there. Just for drill I tried loading one in a HPH-3p-EZ load sabot and even a Harvester Crush Rib - as I expected they were tighter than a regular 458 sabot.
There are only two options that I could offer one is a session with JB bore paste to smooth and clean the bore. I do this with all my rifles, especially new ones. The treatment is fairly easy...
This is a video of the treatment...
http://www.brownells.com/.aspx/lid=13084/guntechdetail/Using_JB_Bore_Paste_and_Kroil_with_the_VFG_Bore_PelletsThe video is really more extensive than you really need to do but it give you the idea. Myself, I use a regular loading/cleaning jag and a tight fitting patch. Just regular non-Teflon bore oil instead of the Kroll and work the barrel from top to bottom.
This is a short paragraph that I wrote of another person with the same problems...
Yes, you might have to double your patch to get a tight fit. But I ran an oiled patch with JB's smeared on the sides...ran that side 25 strokes top to bottom (important) then flip the patch over and run another 25 strokes. Did this 4 times then clean up the bore and see what you might have accomplished... On my Triumph I think I ran this operation twice.
Hope this helps
mikeThere is one other alternative... well actually two... The first one I might suggest is that Lehigh offers an alternative .458-300 grain bullet built for 45-70 rifles. This bullet is built to tight specs and is slightly less in diameter but longer in length. In my case it is very easy to load so easy in fact that I use regular .452 sabot to load it.
This is the bullet...

Because this bullet is over an inch long it needs every bit of the 1/28 twist it can get so it needs to fit snug. If the regular bloodline is TIGHT then this one should be snug in the 458 sabot in your barrel.
mike
Sent you both a PM