After sizing and then wet-tumbling the cartridges I let them dry in the oven at 100°C / 212℉ for an hour. When I measure the cartridges afterwards they have shrunk from 50.95cm to 50.80cm (1.5mm) Testet with cartridges from the same batch. Cartridge that hadn't been dried in oven was 50.95cm and the ones who had been dried was 50.80
Is this normal?
Kristian, I've got to admit I got a few chuckles out of this too when I first read it,
but we all have to start the learning process somewhere and asking questions is a good way to do it... so here is a straight answer with a few corrections along the way:
The case measurements provided are expressed in mm,
NOT cm. Look closely at your calipers. If they are the digital kind, the button to toggle between Imperial and metric measurements is labeled "in/mm". With this in mind, the math below highlights the length differences you found.
50.95 mm - 50.80 mm = 0.15 mm size reduction = 0.0059 in. ( you can round up and just call this 0.006 in. )
That said, your shrinkage has nothing to do with the oven drying process and everything to do with peening the case mouths during the wet-tumbling process. Your tumbling media (assuming you are using steel pins) is effectively impacting or hammering your case mouths and slightly denting/deforming the mouths as suggested earlier by mijp5.
To completely convince yourself this is what's happening, run your experiment again. Size the cases as you did before, wet-tumble half the cases and simply rinse the remaining half to remove the sizing (no tumbling). Then, oven dry both batches at same time while still keeping the two lots separate from each other for later comparison. Once dry and cooled, measure cases from both lots.
You will find the cases run through the wet-tumbling process are shorter than the ones you simply rinsed. This should convince you that the oven drying had nothing to do with the observed shrinkage... or you can just take my word for it!
Two Thumbs Up!
