As mentioned, the hammer should not quite touch an uncapped nipple face. The color case hardening is a thin skin, and the softer metal behind it will emboss from the nipple face, making a ring.
The Ruger Old Army is set up to miss contact by a few thousanths to prevent damage to the hammer and nipples, and theoretically would allow dry firing.
If the ring embossed on your hammer is a full circle, (of even depth) the face angle is correct, if it is hitting only on the top, (most likely), the face angle of the hammer nose is wrong and should be corrected. This is most often the case with a Pietta Remington, which also tend to strike too deep.
If you do modify the hammer, do so with the nipple brand you expect to use, don't set it up with factory nipples, then change to Treso/Ampco or SS, change out the nipples first, then tune the fit.
Another note, if you have spare cylinders, make sure they are all fitted to the hammer face. I have found some cylinders of different age to vary in the depth of the nipple seat, and had to tune the revolver to the deepest cylinder, and let the shallower cylinders get struck too hard.
Do not shorten the nipples, it's hard to get them all the same, and they must be so, and they may not fit the caps properly after shortening them. A hammer can be replaced if necessary, or the hammer or frame relieved a bit to let the hammer nose in a bit more if too much was taken off.
Treso/Ampco nipples are bronze and can be damaged by dry firing or too deep a hammer stroke. Stainless Steel nipples are more likely to damage the hammer. In either case, the best setup is for the hammer to not quite touch the face of an uncapped nipple.