Tom:
I'm confused -- this is a permanent thing, sometimes it's just more noticeable -- what are requirements for the muzzleloading-only season?
What are muzzleloader requirements for cartridge gun hunting season, if different from muzzleloading-only season?
I ask because several years ago, Idaho permitted such things as double-barreled rifles.
Mamaflinter is, as usual, correct. But, having only one shot available, make it the biggest, most refrigerator-like projectile you can stand. Notice, I did not write "handle." For one shot, you can stand fearsome loads.
And when you practice, just put a sandbag between you and the rifle's butt. I got my elk in Washington using a .50-caliber RB. Next year, g-d willing, I'll use a .62- or .68-caliber rifle. Most of the pieces are in a box in my garage now.
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Where are you going to hunt? Around Grangeville, or perhaps Lemhi county? Walking in, driving in, or horse packing?