Never gacked a woodchuck, and though I shot a lot of rockchucks in my youth, I never did so with a muzzleloader.
But I HAVE shot a bunch of squib loads in an array of 54 cals including TC and Lyman, and one fast-twist unmentionable. I also have a 58 caliber Lyman 1:48 and shoot squibs in that. I do a bunch of large caliber small game shooting, so my experience with them spans quite a few years.
My standard 54 caliber squib is 30-35 grains of 3f under a patched RB, irrespective of twist rate. They all seem to like it. With all those guns sighted in at 75 yards with full power loads, I've never had to touch the sights for shooting squibs. They all print more or less right on the money at 25 yards.
I'd hesitate to go below 30 grains on any barrel with a patent breech though, as that's about what it takes to fill the breech and avoid an air space. Maybe not a problem at such low pressures, but I'm still not willing to play the game.
My Lyman 58 for some reason just seems to like 40 grains of 3f better with patched RBs. Accurate as any of the 54s, but with that little extra nudge.
Based on clanking a lot of small game out to about 35 yards with those loads, I have zero doubt that it will do the same with even a tough old woodchuck, and I'll venture than you never recover a single ball from one. It's a big hunk of lead that hates to stop for any reason.
I have noted that the slower loads are more prone to ricochets off hard surfaces. The hotter loads just splatter, but the slower ones can get a good bounce going with an angled shot. Soft soil should be no prob.
Woodchucks sound like great fun! :thumbsup: