The Power belt that I showed the picture of is legal. The bullet is completely made of lead. It had a plastic base but it is not a part of the bullet it is a gas check. They are accurate to a point but on game they suck.
I harden my bullets a bit for better accuracy and better performance on game. I have only found a couple of those bullets. One was on a cow elk that I shot in the shoulder at 60 yards. The bullet was found under the skin in the flank. It penetrated over 3 foot of elk before it stopped. The bullet I showed before crushed the shoulder of a bull at 248 yards and broke the spine dropping the bull in its tracks.
This bullet is my paper patched 45 cal RCBS 11mm rifle bullet I make. These are a bit harder than the 50, they are actually a Lee 500 S&W bullet I make my self. This bullet hit a buck mule deer in the flank and plowed forward to be found in the neck close to the head.
Those bullets start out at 408 grains. I grade them +- 1 grain.
That bullet still weighs 408.2 grains.
I have been hunting with BP off and on for over 40 years. 15 years ago I got really serious about what I wanted in my guns and loads. That was when I put together my first Hot Rod guns. and the Bullets came after that. The first animal known to be killed with paper patched Lee 500 S&W bullet was this buck.
After that one, I have killed many deer, elk and antelope with them. There are a ton of guys that have followed my loads and have killed piles of game with these bullets.
The reason that these bullets are superior are first, the paper makes the load more consistent. The paper protects the barrel from the lead, and the paper protects the bullet from the barrel. Naked bullets leave a trace of lead after every shot. It might be very miniscule but it does leave some. The next one leaves a little more and the next a little more, this will affect the accuracy in time, Plus you add in the powder fouling. I'm not saying that naked bullets can't be accurate, they can. But it takes more work. You have to clean better. You have to have better lube. And in my opinion it takes a better mixture of lead.
I had luck with naked bullets but the paper patched bullets have proven to be more consistent.
Paper patching does take work developing a load just like any other bullet.
I have found that with 80 gr of Pyrodex P with a over sized wool over powder wad .620 for a 50 cal, in a Green Mountain barrel the load shoots very well. Green Mountain used to make and I guess still does make barrels for side lock guns and they make the barrels for Knight rifles. My knight MK85 inlines shoot the same as my side lock guns. I just like my sidelock guns better. These barrels shoot very very well with my load.