pressure ring expansion - 24hourcampfire

I would think that if the chamber is 0.5140" just in front of the belt then the cases would measure 0.5140" after firing. Upon firing the case expands to fill the chamber, right? Even with lighter loads, right? So why do the lighter loads show a smaller pressure ring diameter than the hotter loads? Is it that with the hotter loads the case does not tend to spring back, that its ductility has been overridden by the high pressure? Thanks, Rufous.

Dr. Howell will I am sure give you more detail and a better explanation but I would say that is exactly what you will find, the hotter load will take the brass beyond its yield strength and so will not spring back as much.

Both the chamber and the case expand with each firing, then contract after the chamber pressure is back to atmospheric level. Pressure curves recorded with strain gauges above the chambers show that the chambers haven't completely contracted when the bullet is well beyond the muzzle. (The chamber is still expanded, so the strain gauge is still strained, so the electrical conductivity of the gauge translates that residual stretch as residual pressure, even though all the powder-gas pressure and the air pressure of the location have equalized.)
 
 It's conceivable and may even be very common that the cartridge expands to diameters greater than the "at ease" chamber's dimensions at those points -- since the chamber is also somewhat larger at that particular moment, expanded by the pressure that's expanding the case.
 
 I remember well how frustrating a new balloon was for me when I was too small to blow it up. I could blow it up only far enough to reach what I now know to call its elastic limit. When I quit blowing and let it go limp, it went back to the same dimensions it had when it was in the bin at the store. So I had to have an older person blow it up for me, enough to stretch the rubber beyond the limit that I couldn't blow it past. After an older person had stretched it beyond its elastic limit, I could blow it up enough to burst it. Also, after someone had blown it past its elastic limit, the balloon had two new properties that were obvious to even this preschool country boy:
 
 It did not return to its original dimensions (or appearance).
 It was not as strong.
 
 Brass cases expand in gun chambers in both of these ways. Weak "little boy" pressures expand the brass a little but let it contract to its original dimensions. Open the breech, and it falls out.
 
 Near-maximim and maximum pressures expand it so far that when it contracts with the relaxation of pressures, it can not contract enough to resume its original dimensions. It may be smaller than the relaxed chamber, but it also may not. It may fall out of the chamber, or it may stick slightly but extract easily with the mechanical advantage of the bolt's extraction cam.
 
 Over-maximum pressures expand the brass so much (while they're also expanding the chamber, remember) that it can not contract enough to be smaller than the chamber when the chamber contracts to its original dimensions. And extremely over-maximum pressures can even expand the chamber so far that it can not then contract to its original diameters. (When I was Editor of a couple of well known magazines, one of my well known writers did exactly this with one of the super-hot loads that he was well known for. The expanded case could not be extracted from his jugged chamber. And a local gun-maker had a dickens of a time unscrewing the barrel from the receiver.)
 
 This is, I'm sure, more "answer" than you expected, but I've taken this opportunity to explain the goings-on in the case and the chamber for the benefit of anyone else who's reading this thread with other questions (similar, related) in mind.

Ken, thanks for the thorough answer. Just what I was after and beautifully explained and illustrated (balloon).

Ken,
 
 I have never heard a better explanation of how a case gets worked in a chamber during a firing sequence.
 
 The balloon illustration was excellent!
 
 Thanks,
 Ted

DR Howell
 Started to answer this last night and knew the right term for failing to return to its original size, historesis, and ALMOST everything. I am glad I waited and I truly sat here and grinned at your wonderful example.
 
 Thank you once again.
 art

Some of our friends around the fire here sort of blow up and have trouble getting back to right size,too.
 
 "preciate the answer,Doc.