I have a service six that is a square butt version of the speed six. The only ammo I have fired in it is 125 gr cast at around 980 FPS and it shoots to point of aim at 15 yards and a bit low at 25 yards. I have also shot 158 gr cast bullets at about 850 fps and they shoot to POA at 15 yards as well and a bit high af poa at 25 yards. I do not buy factory ammo so have no idea about it.