I have only had the problem you describe with snap caps on ejector model guns (these models will feature split extractor rods, but I've had no such problem with Smith guns which have an additional pin to prevent sagging/tilting of the extractor stems). As your gun is a manual extractor model, it could be that the shell rims on your snap caps are just a bit smaller than a standard shot shell; try a couple of fired cases in the gun to see if they also slip over the extractor rim cut before consulting a gunsmith.