They are only similar to a .38 if you use weak powder. If you use a sporting grade powder such as Swiss, Olde Eynsford, or Triple 7 you can surpass the performance of a .45 Colt.
Even my weak load, 8.3 grains Unique and a 255 grain flat nose makes over 400 ft lbs and does it with a nice, LONG 255 grain chunk of lead. Now, a compressed charge 777 pushes a 220 grain cast conical up in the 700 ft lb range in the ROA with the Ruger cylinder, but the .45 Colt surpasses 1000 ft lbs from a 7" Contender barrel, .44 magnum territory, with a 300 grain Hornady XTP over a heavy charge of 2400 and I COuLD probably push it a little harder if I wanted, too. I really don't want to. Nothing I can't kill around here with that load and I don't shoot it much at targets, stick with the lighter load in my Ruger Blackhawk..
Even though cylinder volume is similar in the '58 vs the ROA, I've never tried to push it like I have the ROA. The ROA is a very strong gun. I have no reason, really, to stress it OR the '58 with this heavy a charge, just wanted to see what it'd do. If I hunted with a cap and ball, however, I'd use this charge in the ROA. One problem, by the 4th round, the bullets are pulling enough in the cylinders to start interfering with cylinder rotation. Don't have this problem with hot .45 loads in my Blackhawk or my TC Contender.
In my particular '58, one of the 5.5" Piettas Cabelas made the mistake of putting on sale at $179 once..
..a charge of 30 grains Pyrodex, cornmeal filler, and a RB seems to be best for accuracy giving me 2.5" groups at 25 yards, pretty respectable. Conicals don't work as well, nor do heavier loads. It seems to prefer the RB over 25-30 grains Pyrodex. Full power loads push 4" groups and the conical opens that to 5". I prefer the better accuracy of the 30 grain charge and, well, I don't hunt anything, but paper and the occasional chlorox bottle with it.
I do keep it loaded hanging from my bed post in a very Hollywoodish buscadero rig. That's partly for looks, just looks cool, but also, it's the first gun I could reach should I need it. My .38 snubby is in the beside drawer or in my pants pocket by the bed and my shotgun is a couple of steps from the bed. I'm very confident the gun will go bang. I seal it with clear nail polish. Recently shot it after hanging there for over 9 months. It fired like a cartridge revolver. I also have no doubt that it carries plenty of punch for an intruder in the very low possibility that I might need it for that.
I agree with kbbailey, I love this thing.
It's handsome, shoots GREAT, but I keep it loaded to its sweet spot for accuracy and the charge is easy on the gun. It's all steel, so I don't need to baby the thing, just see no need to stress it. I'm still kicking around the thought of getting a .45 ACP conversion for it, but I think I've about gotten over that desire. I'd saved some money up, but then I had to buy two registrations for a car and a motorcycle, so I spent what was left on an egg incubator I've been wanting for the chickens. And so it goes.....LOL