I recently found out that there are 2 different models of the Numrich Buggy Rifle.
I have what may be the older version:
But this is the other version also by Numrich:
I do know that my Numrich barrel measures .45 from groove to groove rather than land to land, so a .433 ball and .018 patch is the preferred round ball load in mine. I read about this preference for .433 balls in the back reference section of the Dixie Gun catalog before I found out for myself. I don't know if both models have the same bore dimensions or not, though they may. Which model do you have?
I have loaded ~.451 projectiles into mine but they were
very tight and needed to be hammered into the bore with a mallet. So I stopped trying to shot conicals out of it. IMO it's safe, but good accuracy is doubtful.
You might have better luck if you have a different size bore or use a different style or diameter of conical , but the TC Maxi's usually measure .451. Maybe something like the 180 grain .45 Buffalo Ball-et (for pistols) might be one to try, it's .451 but it has a cross hatch pattern around it's side and may load looser than what I used. But I've never found them to shoot very good out of my rifles.