Wild Bill Jr
You need to understand a few things about twist rates. The length of the bullet used is what controls twist rates but the caliber also enters into it. To get the right spin on a bullet for it to stabilize brings the load to to play also. for instance the 54 cal you mention is one of my calibers a 1-48 twist works good with a 110gr load but if you use a 1-66 you may be able to get 130 gr load of RS to group tight with a PRB the PRB in a 54 is very effective for North American game. The guys are right about conicals there are some available for a 54 but the range of choices is not near as good as for a 50. I will include some info on twist so you can get a feel for it.
Twist = 150 X D2/L
Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant
Twist = C * D2 / L
The original value if C was 150, which yields a twist rate in turns per inch, when given the diameter D and the length L of the bullet in inches. This works to velocities of about 1800 f/s; above those velocities, a C of 180 should be used.