mtmanjim,
Mine is a fast twist target barrel from Ed Rayl. I had a lefty GPR shipped to him and he used the parts from the fifty barrel. I say target but in fact it's no different from shooting a 40-65 Winchester as far as "performance" goes. It'll kill anything I'll ever hunt. At rabbit and squirrel distances it shoots tight patched ball well enough for heads shots. But, being a fast twist it's cantankerous and persnickety about cleaning and loading.
Now, if I ever get another forty barrel it won't be fast twist or slow twist. It will have rifling suitable for patched round ball and to shoot .41 pistol molds cast from soft lead loaded as maxi's. As far as performance goes in more modern terms it'd be about half way between a 38-40 and a .41 magnum, plenty of available weight and velocity for penetration while still being easy to load patched round ball with good accuracy.
Any how, that's all I know forties unless I got started on what doesn't work.