mdbrown
Gee! i can not believe the Big Horn is that choosey... Since we can not use a sabot either - I shoot 460 grain Bull Shops during that season. They are a really good projectile... I shoot them from a GM barrel also.
You could get some 460 No Exuses out of Utah in couple of days - they are pretty close to the same thing - just a different lube....
Where are you at in Idaho - never mind I guess since you said Mule Deer you probably are in southern Idaho... I was thinking if you were up north I could get you some Bull Shops and you could try them....
And very importantly - Welcome to the Forum - this is an excellent place to be.