I found a couple of inexpensive .50 bullet sizers at a gun show. I never used them but IIRC they measure around .505 - .507 at the mouth. A bullet would need to be pushed through one to determine its inside diameter measurement.
The rifling grooves on most TC sidelocks must be at least around .006 deep or so per side which would make the groove to groove bore measurement about .512.
I doubt if a flat base bullet will obturate in the same fashion or as easily as a hollow base or minie type of bullet.
Nonetheless, if rapid reloading is too difficult in the field then I recommend using a patched round ball for more rapid reloading for a follow up shot. Or a loose fitting conical like a Buffalo Ballet will fill the same bill.
The main problem with loose fitting conicals is that if the muzzle is carried pointing downward then there's a risk of the conical creeping up the bore and coming off the powder charge.
I suggest carrying the gun's muzzle in an upright position through the woods to help prevent ringing or bulging the barrel that could result.
That goes for all conicals really since once they're engraved and rammed they do tend to loosen up.
Another option would be to stuff an over powder card on top of the conical to help keep it down on top of the powder. A ball of newspaper can be tamped down and compressed to serve the same purpose too. But then that loading variation would need to be tested to insure that it doesn't alter the accuracy of the bullet.
The last suggestion is to use a wooden mallet to gently tap tight fitting conicals down the bore. By the time they're part way down the bore they become easier to ram.