The 577 Snider is notorious for being loaded with bullets undersized compared to the barrels. The first ones were conversions of the 58 caliber Enfield, which used the 58 caliber Minie bullet that was considerably smaller than even the bore diameter (much less the groove diameter). The reason the Minie bullet worked in the percussion Enfield was because the black powder charge caused the hollow base's skirt to obturate to fit the groove diameter, with room for black powder residue. The original Snider bullet was likewise smaller than bore diameter and also depended upon obturation of the bullet to fit the barrel. Smokeless powder will not satisfactorily cause this obturation (with safe charges, at least). I found that even to cause ignition of light NFB loads in my converted Enfield I have to mechanically expand skirts of Minie bullets to at least .620" with a self-made punch and ring to limit the amount of expansion. This necessitates leaving the cases " as fired" and with a neck diameter larger than a "factory" cartridge, as observed by AGS above.
Mike