@Slamfire how does brass get stretched as the bolt closes?
You are correct, the case does not get stretched on chambering, that is a misstatement by me. It will be stretched during firing if there is clearance between the case shoulder and chamber. This is inevitable at some level. The firing pin hits the primer, shoves the case a bit forward, the primer comes out of the pocket and positively pushes the case forward. Then as pressure increases, if the case shoulder grips the chamber front, the side wall has to stretch for the case to touch the bolt face. That is, if there is enough pressure to stretch the case sidewalls. Professor Boatwright estimated the pressures to stretch the thicker, near case head sidewalls of a 243 case was around 25,000 psia. Incidentally, this is how the "famous P.O Ackley" lug less 30-30 trick worked. P.O Ackely removed the locking lugs from a 30-30 and fired one of this Ackley Improved 30-30 cartridges, and the bolt stayed put. If anyone cares to notice, P.O Ackely never ran that same slight of hand with an AI 30-06 where the pressures are above 65 K psia. He could have milled off the lugs of a 30-06 bolt gun, fired a full power Ackley Improved 30-06, and at those pressures, the case would have separated and the bolt would have blown out.
Ackely did not pressure test this test, nor did he ever release any of the bolt thrust data he had. In the first issue of Handloader, Vol 1 1966, Ackley writes an article titled
“Wildcat Pressures” .And in it is a picture of bolt thrust test fixture that Ackley made. Whatever data he collected Ackley never released, obviously because Ackley's straight walled cases did not "reduce bolt thrust".
You can look at Varmit Al's page, he calculates bolt load for various case frictions.
http://www.varmintal.com/a243z.htm
at an extremely high case to chamber friction, and "low pressures" in a fixed breech gun you can get the case to lock in and not stretch.
Increasing the Coefficient of Friction to 0.55 the brass case head never does contact the bolt face for the 35,000 peak chamber pressure load. This condition would have the primer protruding slightly.
Varmit Al also shows that a high case to chamber friction actually increases the stress through the case head. As he says:
For a very rough chamber with a 0.55 Coefficient of Friction the movie shows the case stretching and thinning where case head separations usually occurs.
The one part of the case you do not want to stress is the case head. It sticks out of the chamber. You can tear the sidewalls off, and all that will happen is a jam. But if the case head ruptures and gas gets into the action, these sort of failures happen
A high case to chamber friction will result in ripped rims with gas guns, and in fact, with most of the mechanisms on the market.
@lysanderxiii is correct, the pressure curve I posted for the Garand timing only shows the pressure inside the gas cylinder against time. Springfield Armory was unable to get the chamber pressure system to work during the test. However, to make my point about gas gun cases stretching, I will show a "generic" 308 Win pressure curve.
The gas cylinder pressure curve
generic 308 Win pressure curve from an AMCP pamphlet
notice the time at unlock for the Garand gas system, 2.3 milliseconds, and notice the pressure at 2.3 milliseconds in the inset of figure 4-5. That pressure is under 500 psia. So when the Garand unlocks, there is around 500 psia pressure in the chamber, and that will fix the front of the cartridge to the chamber at the time the bolt unlocks and moves to the rear.
It is worth going to Chinn's Machine Gun Book Vol IV, take your pick of where you want to download it
http://www.milsurps.com/content.php?r=347-The-Machine-Gun-(by-George-M.-Chinn)
http://www.ibiblio.org/hyperwar/USN/ref/MG/
and look at the blow back section, and the gas gun section. Chinn clearly shows that unlock occurs while there is still pressure in the chamber. The pressure is below the rupture limit of the brass case, and it is there to help "pop the case out" of the chamber. Chinn calls it the residual blowback effect.
Chinn provides these images in Vol IV
This residual blow back effect is why gas guns stretch brass badly if there is high case to chamber friction. Such as the stretching that occurred with these cases extracted from a fellow competitor's rifle. These were fired in a M1a. Bud fired dry, unlubricated cases in his rifle and tossed the brass at the fifth firing . It was not wise to continue shooting ammunition just at the point of case head separation in matches, as malfunctions ruin scores.
I did a crude measurement of my lubricated cases to see what dimensional changes occurred.
prior to being fired these cases were all sized down to the minimum "Go" dimension.
All of these cases were fired lubricated. I rubbed Johnson paste wax on these cases, under the temperatures and pressures of combustion, the wax melts and becomes a lubricant. Just as the ammunition in the Pedersen rifle. Pedersen used a wax lubricant on his 276 Pedersen cases.
https://www.forgottenweapons.com/m1-garand-development/us-pedersen-276-rifle/
In his patent, Pedersen clearly claims its to be a lubricant.
Process of coating cartridges
Patented Nov. 4, 1930 JOHN DOUGLAS PEDERSEN, OF SPRINGFIELD, MASSACHUSETTS
http://www.google.com/patents/US1780566
In the present invention, the material for said coating has been so chosen as to perform the additional function of acting as a lubricant for the case of the cartridge, both for facilitating introduction into the chamber of the gun and the extraction thereof after firing.
What you can see from my gauge measurements, is my lubricated cases are longer by about 0.0045 ish after firing. This growth in length is not from side wall stretch, it is from the shoulder moving as the case is withdrawn. I would have had sidewall stretch if the case neck and shoulder were stuck to the chamber, but since I lubricated them they did not adhere. Instead, the shoulder was deformed, moved I am going to say, during extraction. The pressure had to be high enough to cause the shoulder to change shape during extraction, in order to cause that psuedo case length growth.
Anyway, given the cost of 308 Win and 30-06 cases, running about
fifty cents to a dollar a case, I think it makes economic sense to lubricate Garand cases to make them last longer. Any bug a bears about lubricated cases increasing bolt thrust and causing catastrophic damage to the weapon are clearly over stated scare tactics
And they all go back to an Army Ordnance Bureau coverup of their defective low number M1903 rifles. These rifles were made in Arsenals where temperature gauges did not exist in the forge shop, nor heat treatment furnaces. Temperatures were judged by eye. Easily a third of all low number M1903's have burnt receivers and bolts, and at the time they were exploding on the firing line. Instead of admitting that their rifles were defective, Army Ordnance created a coverup, claiming grease and greased bullets were unpredictably raising pressures to catastrophic levels.
Greased bullets never bothered Swiss rifles.
Captain Crossman demonized greased bullets as dangerous in his
Grease article in the 15 April 1921 Arms and the Man article. He knew that the Swiss were greasing their bullets, and dismissed it as something the Swiss did to shoot holes in cheese!
Some nations still flirt with it. The Swiss cartridge, with its boat-tail bullet, all neatly packed in the cardboard clip, has a little grease-or soft wax, to be correct-where bullet joins case’ but we do not know whether this is regular issue stuff for the Swiss private soldier or whether it is extra special stuff for shooting holes in the Swiss variety of cheese. 
