You could always take a page from my friend Rod's testing - he uses 1/2" ply, water jug (milk jug full of water) then 1/2" board, then jug, 1/2" board then jug, etc. He's found over the years, using the same stuff in comparison against a myriad of rifle calibres & loads, that 2 boards and 2 jugs is equal to descent results with rib shots animals including elk, deer, bear, etc from 6.5's to .375's. More is always better - never forget that. You may be surprised at how well a round ball does. 000 Buckshot almost makes it, not quite - that's .35" to .36" @ 1,300fps MV which is a squirrel load in a .36 cal. ML rifle.
As to a solid copper projectile in a cloth envelope - well, you could try it. Seems to me the Barnes bullets for .45 cal sabots are .358's in around 125gr. to 150gr. I'd as soon make a bismuth ball work. Because it's solid copper, it's much longer than a lead core with guilding metal jacket. That slug needs a much faster twist than your barrel provides - faster than 20", in fact about 18" would be about minimum for around 1,500fps. If you could drive them at over 2,000fps, you might get away with 20". That would take about 100gr. of 2f - maybe 80gr. 3F might come close. I suspect your rifle barrel has a 60" twist, but could be 56" or 48" as well - all very much too slow for solid copper conical pistol bullets.
Don't take my work for it though, I'm a bit biased and stranger things have happened- maybe they'll shoot. They won't be stable after impact with an animal, but they may shoot with some modicum of accuracy. You'll need a short starter with a cupped nose that closely fits your bore, to hold the bullet's nose centred. This is also the problem with shooting picket bullets. The ramrod shouldalso have this bullet-nose shape on the tip, to keep the bullet centred as it's pushed down the bore. After pushing the bullet down into the bore a ways, rotate the starter to ensure the nose if centred with the bore's axis. Lot's of trouble for no gain me thinks.
