I know someone who just bought a NIB Ruger OA, a stainless R&D 6 shot .45LC conversion cylinder and some UltraMax .45LC ammo with 250 grain RNFP lead bullets. The box of ammo does not say that it's cowboy ammo
anywhere on it but the UltraMax web site shows that it is with a velocity of 725 FPS.
After firing it for the 1st time he sent me a photo of the target to show how well it shot. I realize that every ammo and gun is different but this UltraMax ammo shoots and works well.
http://www.ultramaxammunition.com/cowboy.php