Hi Jens,
I have finally gotten to check the twist on my Ingram .500BPE and the barrels are 1 turn in 41" as best I can measure.
Barrels are 28" and the rifle weighs 8lbs 10oz. Proof marks indicate some time after 1887.

interestingly enough my rifle does quite well with a 440 grain paper patch bullet and 58 grains of H4198, backer rod, and Federal 215 primers. Have not chronographed that load.
The bullets are cast of 1-20 from a Tom Ballard adjustable mould and are patched to ~ .509" diameter with Onion Skin paper, thin smear of SPG lube on the paper patch. I put one .515" punched out "milk carton" wad under the bullet to protect the base and paper patch.
Groups (2 rights, 2 lefts) run about 2 1/2' at 50 yards. I don't have a suitable 100 yard setup, perhaps the shots would spread apart at that range?
I more often shoot 365 grain gas check .510' diameter grooved bullets with the same powder charge.
- Mike