The old Custer bumper stickers used to say "Custer got Siouxed". The 50-70 is a fine shooting rifle. My current 50-70s are a rolling block that Higginbotham built for me on a 1902 action and a restocked, rebarreled Belgian Comblain. I suspect that one barrel is a Green Mountain and the other is a Badger and both are fast twists that will easily handle 600 grain bullets.

I have just started gearing up for spring bear season.  It would please me to no end to take a bear with my Sharps 50-70. Trying to work on a load that has plenty of punch but not too stressful on the Sharps action using smokeless powder.  This is an interesting rifle that I believe has been converted from a percussion lock.  It has a Montana C. Sharps barrel so I assume they did the conversion work too.
  This is a hunting rifle.  It is neither rifle nor carbine with a 26" tapered round barrel it is right in between the two.  It is a perfect weight for packing. The tapered 26" barrel gives it enough weight the frisky loads are not too punishing but also light enough that its easy to carry.  Anyone that has packed a long 30" to 34" heavy barreled BPCR for hunting will appreciate that.  
It is set up with a globe front sight with fixed heavy post and pin head. No inserts to loose at the worst possible moment.  There is a filler in the dovetail for the barrel sight. I removed the original Laurence sight because with the tangs sight down for using the barrel sight the top of the tang sight gouges my thumb so I decided to go with the tang sight only.  Its a mid range Lee Shaver economy Soul sight. No need for a more expensive target sight on a hunting rifle. The Lee Shaver economy model is a very good robust sight well suited to field use.  No Hadley eye cup either just a single fairly large hole the will cover most light conditions.  This rifle is set up to be a serious no frills hunting rifle and that it is.
   My favorite load so far is with a bullet I designed using the design program that was offered by Dan Lynch when he was making custom molds. 
The design is a 525 grain wide flat nose gas check.  I actually designed the bullet specifically for a Model 1886 Winchester in 50 Alaskan. The bullet proved its metal in that rifle for hunting moose.  The 50-70 wont get near the 1750 fps I was loading the 1886 to but at about 1250 fps MV I believe it will do the job on a bear out to about 150 yards or so.
   At 100 yards I am keeping shots in about 3" but I feel that a younger set of eyes can better that. 
The load with the 525gn Mountain Molds is a duplex smokeless powder load.  It is put up in fired but  un-sized cases and primed with CCI 200 LR primers.  The kicker charge is put in first with a Lee .7cc dipper of Accurate Powders LT-30 then the main charge of 40 grain of WC-870 is thrown from an RCBS uniflow.  WC 870 is the same as H-870 and Accurate 8700.

I shoot a #5 rolling block in 50-70 with a plain base bullet of about 512 grains when cast with 16:1 alloy.  Velocity in my short barrel is 1380 fps with Re 7 powder and shoots 1 1/2 moa at 100 yards with a three shot group.  I shot a buffalo two years ago and recovered the two bullets.  They lost virtually no weight and slightly flattened but nothing like the expansion you are getting.  I think you will have no trouble killing a bear if you can get into range.  I tried softer alloys and the rifle did not shoot near as accurate as it did with 16:1, and I read that was what they used for gg bullets back in the day so stuck with them.  Tom

As to accuracy with BP loads in this rifle or any other 50-70 is not an issue. BP may well be the most accurate powder for the cartridge. There are two reasons for me using the duplex smokeless load and they are clean up and cost.
Everybody that uses BP in cartridges knows the clean up is two fold. One is the rifle and the other the brass. Its not hard but it is an extra chore and at the end of a chore filled day  eliminating one or two seems like a good idea.
The other of cost is becoming an issue in these times of inflated prices. Using the slow surplus powder for a main charge is absolutely the cheapest way to load the BPC cases.
I guess maybe I cant say that for certain now because I have seen the cost of these surplus powders going up too. If though as I have a person stocked up when they were a fraction of the cost of commercial canister powders then for sure you can do a heck of a lot more shooting for the same money.
Another appealing aspect of this type of loading is that they stay in the same pressure and velocity range as with BP.