You will have to do a chamber cast and you may as well drive a lead slug down the bore.

I would start with an empty .45-70 case to see if it fits.

The follower has been modified to probably feed a non-original cartridge.

Have fun with the new toy.

You can always get your money back by 'parting it out'.

p.s. The original 8mm barrel would have been covered with a sheet metal jacket.