I have been shooting old Winchesters for 30 years and have a 1876 Winchester carbine that the 45-75 round was developed for black power use.
This rifle has been to the proof house in London and I decided on my visit with the proof master to go for a certificate of non proof.
I happily shot on a regular basis with a slow burning powder. I would need to hunt in the cuboard for the type.
I have friends in the US who do the same.