I have quite a few black powder pistols. The one that will shoot straight right of the box is the Ruger Old Army. I have the stainless steel one(the only stainless gun I have). I hate the idea of stainless steel guns,but this one is just for practical reasons.
I bought a Remington some guy built from a kit. it would not hit a 5 gallon can 20 feet away. I recovered bullets,and saw that they just barely had rifling engraved on them. Got to measuring around,and found that all the Italian guns had cylinders TOO SMALL for the bore!!
So,I unscrewed the barrel on the Rem. I made a fake barrel about 3" long,with a 1/2" bore reamed in it. I made a reamer from 1/2" drill rod. It was sawn and ground into 1/2 its width to provide a cutting edge. It was tapered some.
I also had found that the cylinders were all a bit off center to the bore. With the fake barrel in place,I reamed the cylinders tapered,and larger than the bore CAREFULLY. The new,reamed diameter was enough to bring the cylinders on center with the bore. I'm not talking LARGE diameters here,just enough reaming to make the cylinders the size of the major dia. of the rifling. I did not want to blow the gun up!!
One last thing I did was replace the front sight with a wide one,and file the rear sight notch wider. I did not want to disturb the original looks of the gun any more than needed.
I made an aluminum mold for my Lyman handles to make the new bullet to fit the reamed bores,and did taper the bullet cavity to match.
The pistol now shoots just as accurately as a Smith and Wesson revolver!! A lot of work,but it was a learning experience too.
I have also taper reamed my other Italian Colt style repros so their cylinders are larger. I made them shoot straighter too,but with less success than the Remington,since I could now just make barrels to screw into their frames. I did not care to make the much more complicated fake barrels they would have required to do so. The Remington is certainly an easier gun to work on.
The only Italian guns I have found that will shoot straight out of the box are the little .31 caliber ones,for some reason. And,I wouldn't say that all of those would be accurate either. I only shot a few,and could get decent accuracy from the ones I shot.