Shouldn't this be in the Top Break section?

Seriously. glad you are basically alright. Lot of metal flew off that in a big hurry. Really would be great to figure out just what went wrong.

I wouldn't trust the weight the assembled rounds theory very much. Cases and bullets can have some variation. Unless you clean all your cases and sort by manufacture and lot they can vary several grains. Same with bullets, especially cast sized and lubed bullets. Most handgun ammo isn't loaded like match ammo. It wouldn't surprise me to find variations of 5 grs or more depending on a lot of variables.