Question about brass resizing and sonic cleaning

[Lefty Trigger]

I stumbled across something really weird with some .308 brass and using my sonic cleaner. I bought about 350 pieces of once fired Remington brass from a member on here and just started using it. I resized, trimmed and loaded about 50 pieces not knowing the chamber of a different rifle I set my die to resize .005 under my go gauge dimension. After firing the brass it did increase in size to a dimension of 1.631, ok now I have a chamber size to go by so after cleaning I was going to reset my die to bump to 1.629. However this is where it gets weird... my sonic cleaner heats the solution to 140 degrees and cleans from 30min to 1 hour depending. Anyway after removing the brass from the cleaner and rinsing them off they all measure 1.628? Is it possible that the hot solution "stress relieved" the brass and it changed size on its own?

 

[rwj]

140 degrees F is not sufficient to cause stress relieving in cartridge brass. Perhaps cleaning removed dirt/carbon that resulted in the same measurement being shorter?

 

[Lefty Trigger]

Seems unlikely that every piece would have the same measurement just removing carbon fouling,... but something had to make the change??

 

[marchboom]

Do another test but with cold solution and see what happens.

 

[piie]

When you say "dimension of 1.631" I'm assuming you are referring to a comparator reading with something like Hornady's headspace gauge?

When did you measure the "pre sonic" cases - before or after you removed the spent primers?

Did you let the brass cool down to room temperature before you measured?