I loaded up about 100 357 cartridges yesterday. I ran short on the 158 grain bullets I normally load, so I opened up a new bulk bag of 135 grain bullets I stocked up on last year because they were available at a good price. I knew the max book load for the powder I was using was 8.4 grains for a 158 grain bullet. I figured that would be a safe load for the lighter bullet, but I checked published data and found I could probably go up to as much as 11 grains. I didn't need to do that, so I was content that 8.4 would be safe. This powder works for 38 Special too and loads safe down to less than 7 grains so I didn't have to worry about squibs or anything regardless of the extra case length.
So I dialed my powder measure drum adjuster down, threw a charge and weighed it on the GEM20. I did this until I was reading several consecutive throws between 8.3 and 8.4. I loaded up the batch and took them out to shoot. I was shooting paper at 100 yards that day. I was losing light and wouldn't be able to use my sunglasses. My clear safety glasses are polycarbonate and not too sharp, so I decided it was safe to shoot paper at 100 without them. I dropped the first clip in the revolver and took a shot. It seemed a bit hot. 1520 fps on the chronograph. I checked the POI through the spotting scope and took some more shots. I noticed the cylinder was hard to turn, and when I went to open it up it was hard to swing-out of the frame. Primers were cratered. They were flowing into the firing pin hole and catching a little. Ejection was a bit stiff. I thought for a minute how that could be since the load was well under max. I figured my drum powder measure might have drifted a bit up during the session but it couldn't have gone up too far. I put my safety glasses on. Then I swapped that clip out for a different one that I was hoping had cartridges with a load that hadn't as much drift. The loads were still hot. Against my better judgment, I shot a bit more but when I had a few hits and had lost the rest of my light I called it. I had probably shot about 15 rounds total and called it quits.
I went to load some new cartridges today. I dumped the remainder of those from yesterday in a tub and started to load up others. The first thing I was going to do was see if the drum measure had drifted. I turned on the GEM20 to zero it and I saw it read 0.000g. I thought, "no way." There's no way I could have loaded 8.4 grams of powder. The error would have been too big not to notice. I had to see anyway. I zeroed a case and threw a charge. 0.842 g.
In case it's not clear, I had mistaken 0.842 g for 8.42 gr. Anyone who uses a GEM20 knows how small the decimal point and g are. So I had loaded the cartridges with as much as 13 grains of powder. I checked in it Quickload and it calculated about 3600 bar or over 52,000 psi. It also calculated the velocity at just over 1500 fps. I'm pretty sure that's close to the pressure of proof loads. CIP PE might be a little higher. I'm very fortunate I didn't pierce a primer or worse.
Besides the error I made reading the scale, I was also foolish to dismiss the signs of pressure because I couldn't think of how I had made any error and so doubted that I had.
