I did reloads, using tripple 7 powder, and 262 gr cast bullets, same result,,don't know where it went
777 is equivalent in many ways, but it burns differently from black powder.
Black powder builds up enough pressure and keeps the pressure curve higher for longer, helping the bullets to obturate into the grooves better.
Those heavy bullets may not stabilize very well, and when you combine that with poor obturation, and you will get the results you're asking about.
Tiriaq is trying to tell you something.
I don't believe your pistol is grouping at all with the bullets you're shooting.
I have antique pistols and shoot them, but not often enough.
I have mostly 455s, but also 9.3mm, and a Webley No2, that will not chamber 38S&W factory brass. I have to run the factory cases through a 9x19 sizing die, use a custom mould to cast heeled bullets, of the proper diameter, .368, to get the little pistol to shoot well at the distances it was intended for, 5-10 feet. It will hold a six inch group at 15 yards.
I load all of these pistols with real black powder, under the original bullet designs, because it's the only propellant that does what the original specs required of the ammunition/pistols.
Most people new to antique pistols are stuck on the knowledge they garnered with modern firearms. These are the mechanisms, with all their faults and nostalgia, that led to modern handguns being as good as they are.
When you're working with 19th century tech, use 19th century tech, and things will work about as well as they did at the time period they were common.