FYI - My 357 max barrel does not fit well on the Topper frame, so I've never shot it that way.
Yeah, I believe the Topper has a shotgun type firing pin which is larger, and the hole around it as well. I have shot some 357 mags on the Topper frame, and some shooting with factory rounds in a 30/30 barrel that fits it as well, but didn't notice any primer probs with either caliber... mostly though I just shot the 38s in it.
When I shoot 357 mag or max I use the 357 Max frame and barrel.