What the OP describes is a common occurrence with many types of ammo fired from many different revolvers. It's just blowback of bits of powder ejecta from the barrel/cylinder gap. In .357s, with .357 mag. ammo, it is more the rule than the exception. It is less common with .38 ammo, but still common. It can usually be reduced by reaming the forcing cone, and is not necessarily a problem with ammo at all. However, ammunition with slower-burning powder tends to spit worse than the faster powders, having a greater volume of unburned granules.