If you have the velocity I can tell you whether or not the handloads you have would be safe to use in a Howell's conversion cylinder. I would not endorse the use of jacketed bullets in conversion cylinders, as they could put strain and wear on the barrel and frame. The barrels were meant for lead bullets, if that's what a barrel is designed to shoot, by all means take advantage of it. Keep in mind you are using a sixgun, not an auto loader. Assuming you currently load .45 ACP you can use some pretty fine bullets for your gun that you could extend the range with. You might even be able to find some cast Kieth SWC's to use in your Remington, provided the twist will stabilize the given projectile. Now that's a very excellent bullet style, very nice looking too.
Of course to be fair I'm related to the outstanding gentleman who created the Kieth SWC, so I may be more partial to that design as opposed to others. But in my S&W they are good and accurate, no doubt about it. The larger meplate of the bullet seems to cut a nice hole in paper, and also seems to increase the downrange performance of the loads. I shot some not too long ago and they were very great bullets.
Just my thoughts on this, I think you could have a target grade weapon with the right bullets and charge of powder. That charge may be 4.5 grains of Unique, or .4.6, increase your loads in SMALL increments and make sure you aren't putting anything more than standard velocity ammunition in there.
I said this before and I'll say it again, load the cartridges for this weapon like a special, not a magnum.
Is it me or am I sounding like Elmer Kieth? I've been reading so much about different pistol bullet styles and weights and how they effect the trajectory, chamber pressure, downrange performance, velocity, and general ballistics. I find the internal ballistics of handgun cartridges to be fascinating, there's just so many outstanding options out there.
Now that I've started loading metallic cartridges I'm loving it!