As to the first part of your question. I can fire new, and once fired cases. By the second firing the case has stretched far enough to start to push the shoulder into the chamber to far. I can fire twice fired cases, but can also detect the pressure issue developing. After the second firing all bets are off, and the shoulder has to be reset.

I have the full resize, as well as the neck resizer. I monitor the cases with the RCBS precision mic. The mic really reveals the progression of the case stretch.

The only theory I've come up with for the dramatic case stretch in the 7mm rum is because of the smaller case mouth on the 7mm vs. your .375

Now to the second part of your question... We were shooting at my friends home. He was loading three cases per load. We had a load that showed some promise, but he wanted to shoot a few more loads just to check. On the second firing of the same load we started to detect a spike in the fps, and the accuracy began to deteriorate. By the third firing of the same load the fps spike was so dramatic that my friend quit immediately. He knew that something was wrong, but didn't know what.

I finally talked to my gunsmith about what was going on, and of course he immediately told me what he thought was going on. Which was the case stretch/headspace issue.

I started with the Hornady Head and Shoulder comparator which revealed his theory to be true. I then bought the precision mic which I like much better.

As I monitor the case stretch with the mic, there is a direct correlation between it, and an increased fps on the chrony using the same loads. That fps increase comes right back down after I reset the shoulder.

I hope this explains your question about detecting the pressure spike with the chrony, and goes further to explain the potential trouble we could have ran into had we not been using the chrony, and my friend hadn't known that something was going wrong.