One of the lads on here, years ago, found they worked right down to his .54 cal, but I do not know the loads he used.
The problems with smaller calibres, is they develop higher pressures & do this much more quickly. With a patched round ball, it may be impossible to get a 16 or 14 bore rifle to develop over about 12,000psi as shown by the low velocities which develop heavy recoil. This is very easily done in .40 to .50 calibres with normal loads that have little or no recoil due to the light weight balls.
Check your old copy of Lyman's Black Powder Handbook for velocity/pressure relationships between the various calibres. With a 'given' granulation of powder, different rifles seem to develop similar pressures when producing the similar velocities.
The larger bores are simply low pressure devices, which logically shows why the larger bores are better users of paper ctgs. The higher pressure rounds cause blow-by and burning of the paper due to the high pressure gasses flowing past the paper wad between the powder and ball. The lower pressures are less capable of doing this.
I shot paper ctgs. up to 1,550fps, which gave the same velocity as my tight (.030") patches developed with the same balls, using the very same powder charge. This shows in this large bore, even with 165gr. 2F, there was literally no blow-by- the paper NEVER caught fire, nor was burnt in any way nor even scorched.