School me on the .40-50 BN cartridge. Would it be suitable for deer and black bear? Could it be rechambered to either .40-70 BN or the .40-90 BN? It is a Shiloh with 26 inch barrel.

Best bet would be to open the 40-50 up to 40-70, but with that 26 inch barrel the 40-90 bn is going to be a bearcat, with horrific muzzle blast and nasty recoil.
Plus the 70 is easie/cheaper to get cases. The 90 you have to form them from 45-100's and trim or 45-90's and either stretch or live with them a tad short.

I would leave it alone. I have a 40-50 BN and it is a pleasure to shoot and very accurate. I would rate it adequate for deer and bear. Ned Roberts claimed he used this number on deer and black bear in NH w/ good results. You can make brass out of 45-70's and there is a good selection of moulds to choose from on the market. I am planning on using my 40-50BN this deer season. My rifle is a Whitney Phoenix. Its your decision, but I would leave it as 40-50BN.

Shilohs will at some point be collectible; so I agree I'd leave it alone in the original chambering. It sounds like a fairly light rifle, how much does it weigh?

Just under 10lbs. Ifit is rechambered it will be Shiloh doing it.

Ranch 13 gave good solid advice,....turning it around through Big Timber will give you a fine, and just as "collectible" rifle, in a good solid hunting caliber,...one perfectly adequate for LARGER critters than deer and Black Bear.

GTC

I still have .40-70 BN cases and a mould I believe.

Why not talk to the good folks up at Shiloh about relaxing your existing barrel's breech up, and fitting a little bit bigger, longer .40 caliber barrel to the same breech up ?
...set your existing forend up to fit either / or, so to speak ?

Might get an edge with a faster twist rate for the heavier caliber,....and love the relaxed pace of what you already have.

These BPCRs are emphatically NOT "Modern High Pressure" systems,...and switching barrels is not a NASA grade evolution.

Glad this isn't my decision,....that caliber that you have is easy to like, on a lot of levels.

GTC

Cost about 750 for a Shiloh rebarrel. Think all their 40 cal barrels are 16 twist.

Spoke to Shiloh and for $100 they will rechamber it to either .40-70BN or .40-90BN