Case head expansion as a sign of excess pressure developing a load for +P chamber?

[Sweet Tooth]

I recently purchased a 300PRC +P from Defensive Edge. The +P chamber is designed to allow increased powder charges by reducing the initial pressure spike from traditional rifling resulting in higher velocities, particularly with heavy bullets https://defensiveedge.net/300-prc-p/ for more description of the +P chamber.
I worked with H1000 to develop a load but have less than a pound left. I was able to score an 8lb jug of RL26 and Magpro on the same day, so figured I'd try something with them. Max charges with my rifle and H1000 were about 7% above the max published load for a standard 300PRC.
My plan with the new powders was to start with charge weights below published max for 300PRC and work up to 7% above that max for my +P chamber while of course looking for pressure signs. In addition to flattened primers, ejector marks, and bolt lift, I thought about case head expansion (CHE).
Measuring CHE accurately seems to be a bit of a challenge. However, if I make a gage block with a series of holes increasing in diameter .0001", I am wondering if I can measure some factory rounds that, for example, fit in the .5722" hole but not the .5721" hole before firing and then theoretically fall in between the .5775" and .5774" holes meaning for this hypothetical brass, CHE is roughly .0003". I would assume I would see similar CHE with the starting loads and eventually see greater CHE with the higher charges.
I'm not trying to accurately measure CHE as our laser cutter I would use for the gage block is not that accurate. However, it is precise enough that I should be able to get a consistent indicator of the relative expansion.
Is "measuring" CHE like this going to give me any meaningful indicator that I'm having pressure problems? I have found no load date for 300PRC+P, so I'm kind of feeling my way forward and want to do so safely yet still take advantage of the +P chambering.
I assume this is somewhat similar to someone working up a load for a wildcat cartridge, so if anyone has any additional information on how to do this correctly, I would love to hear them.

Thanks

 

[Doom]

Reviewing what defensive edge is saying is essentially what Remington did with the 308 by using a longer freebore dimension which gives the chamber more effective volume upstream of the lands. The +P designation is misleading as this really has nothing to do with increasing the pressure allowable for the cartridge.

You develop the load the same way you do with any cartridge. As you work up the load you look for signs such as primer flattening, hard bolt lift, ejector marks, etc.

You will likely go over maximum published loads for the PRC and you are on your own without actual pressure monitoring. Whether you get to 7% over max is questionable but it’s a starting goal.

 

[Steel head]

DE
Isn’t providing loading data for their chamber design?

 

[Doom]

They did a workup which was a typical workup looking for pressure signs with H1000. No pressure measurement.

My Rem 700 has a long freebore. In it the lands are so far away that a 168 SMK only has the boattail in the case if touching the lands. I went looking for pressure signs but gave up at ~5% over book max.

This is one of the cases where SAAMI allows modification of a chamber as long as it doesn’t cause an increase in pressure. I think all the did was taper the lands.

 

[Sweet Tooth]

That all makes sense. Any thoughts or experiences with using CHE as indication of excess pressure?

Thanks

 

[spife7980]

That all makes sense. Any thoughts or experiences with using CHE as indication of excess pressure?

Thanks

Yes but it varies between brands and the brass-chamber match has a much larger input on how the brass grows, not just pressure.
So you sorta have to set up your own tests to create your own parameters for your particular situation to realize what the given measurement falls within your own range of results. Measure it if you want but realize it wont do you much good until much later on. Learn to read pressure properly. https://www.primalrights.com/library/articles/understanding-pressure

Just because a snake oil salesman sold it doesnt mean the laws of physics no longer apply, dont just dump an extra 7 grains in a chamber.

 

[Sweet Tooth]

That was a nice article on pressure signs, thanks. I understand CHE can't reliably used from one brand of brass to another, but I'm hoping limiting the variables and checking to see if there's an increase as I go up with the loads will be just another pressure sign to look for and analyze.