Ive shot several rugers and ubertis with the extra acp cylinder. standard ball ammo, in my examples has hit to the same poa as 45 acp and frequently makes smaller groups. I also use a soft ball load pretty much equivalent to hardball but with lead round nosed bullets.
A friend found a cylinder at a gunshow that fits several of his ubertis with no modification. I don't know how much luck was involved in this.