I inherited an old Colt New Service in .455 Eley. Checking the archives I found one discussion from 2010 that left me more confused than I was before I read it.
Can I just use .45 LC brass? What bullet (diameter & weight) can I safely use?
Not that simple. The 455 Eley was a British service cartridge and dimensionally different from a 45 LC. The case is shorter. However, lots of the cylinders of those British New Services in 455 Eley had a reamer run through them to make them 45 LC. This was done post WW2 when the market was saturated with obsolete military weapons. While this period ad does not have a New Service in it, you can see that the importers of the period were altering military firearms to make them more saleable to the American market. Military Mausers were not 270 Win, and the issue G43 rifle was 8mm Mauser, not 30-06.
So, the question is, does your cylinder accept 45 LC ammunition? If it does, then it has been altered. And is it safe to shoot 45 LC in it, I would say yes. While I don't know, I doubt Colt changed the materials and heat treatment of the New Service in 455 Eley from the 45 LC models. Just shoot moderate 45 LC loads in the thing, understand, it is a WW1 period pistol, which means it is made from plain carbon steels, due to residual elements they could not remove from the steel, period steels are not as strong as the same steel would be today. Also, the frame is dead soft, and so is the barrel. Just mild 45 LC, none of the Buffalo stomper loads.
I would recommend shooting 250 to 255 grain bullets, nothing heavier. And use 454 diameter lead bullets. That's what I do because 0.452 bullets will lead. In my New Service revolvers, 454 diameter is the largest commercial cast I can find. If I cast my own, I would look around at trying even larger bullets, maybe 0.456" . I do not recommend jacketed as I don't know how long the rifling will last with jacketed. Who shot jacketed 45 LC ammunition in 1916?
