Excessive headspace between case and chamber will cause severe case sidewall stretching. This is undesirable and will lead to case head separations, such as evidenced by these 300 WSM cases. The shoulder was pushed back too much.
What happens with cases that are too short from base to shoulder is that on ignition, the thinnest parts of the case, that is the neck and shoulder expand first. They expand to the chamber walls and the friction between case and chamber is such that the case is locked into position. Since the case is shorter than the chamber there is clearance between the cartridge base and the bolt face. As pressures increase beyond the yield strength of the brass, the case sidewalls are stretched until the cartridge base makes contact with the bolt face.
An ersatz procedure to save your cases is to leave sizing lube on the cases. Any lubricant, including oil and grease will break the friction between case and chamber, which will allow the case to slide to the bolt face during the pressure rise and this will prevent side wall stretching. A light coat of lubricant is all that is needed, though Army and Navy machine cannon used grease.
Prior to WW2 a number of machine gun designs were made and the manufacturers did not have the manufacturing technology to maintain headspace for replacement barrels. There were also instances of tolerance stack up, due to errors between calibration systems, wherein the headspace of the ammunition and chamber was essentially uncontrolled. For these situations greased and oiled ammunition was used as recorded by Chinn in his Machine Gun series of books:
The Machine Gun Part V
Chapter 14 Birkigt Type 404 20-mm (Hispano-Suiza) Cannon
page 578--
After further comparative tests in late April 1942, it was again definitely decided by the Ordnance Department that all American-made 20-mm automatic guns continue to be made with the chambers longer by one-sixteenth inch than the British regardless of the employment of the same ammunition. This decision was final as far as American production was concerned, but in no way did it change the British representative's view on the longer chamber's performance.
Oddly enough, the question was again raised, not by the English or our many proving grounds, but by manufacturers of 20-mm ammunition. In testing their cartridges for reliability of action, they encountered a series of malfunctions known as light-struck primers that were all out of proportion for such a weapon. These were not isolated cases, the reports coming in from practically every maker of 20-mm ammunition that was engaged in function firing his products.
Since the munitions companies pointed out that the faint strikes were due to lack of impact on the primer resulting from error in the gun, and not as a result of defective materials or workmanship, it was decided to conduct another test on an extensive scale at Aberdeen. Ninety of the 20-mm guns, M1 and AN-M2, selected from every facility producing them, were expended in this test with all types of ammunition, both from accepted and rejected lots.
A complete record was made of every malfunction during the entire test and the probable causes of the trouble. The engineers in charge of the project in the early stages of this test recommended that two modifications should be made to overcome the serious malfunctions:
"(1) Shorten the chamber one-sixteenth inch, thus modifying it to approximately the British chamber.
"(2) Replace the extractor spring with a solid plug, thus positioning the rounds by means of the extractor. This change would include such modifications to the extractor, the bolt, and the ejector, as were deemed necessary."
page 588—
During war all that can be done is to install and make function as reliably as possible that which is issued. With the mounting of the 20-mm cannon in Navy, planes a series of malfunctions began that could not be properly corrected at the time because manufacture was at the peak of production. The slightest change would practically mean retooling. The most serious problem was the oversize chamber. There still remained considerable variance in dimensions between the chambers of the British and American cannon, even after the latter chamber was made one thirty-second inch shorter
Due to an outmoded agreement of long standing, everything above caliber .60 in the Army is considered artillery and the manufacture of the Hispano-Suiza cannon therefore came under this classification. In other words the production of this high-speed machine gun was done under artillery manufacturing tolerances. The resulting poor mating of parts, coupled with the inherent fault of all gas-operated weapons whereby the breech locking key in the receiver is immovable and the position of the gas port in the barrel is permanently fixed, made it impossible to adjust the relationship between barrel and breech lock to establish head space. Thus the most vital measurement in any automatic weapon was governed by chance in this instance.
An unfortunate discovery was that chamber errors in the gun could be corrected for the moment by covering the ammunition case with a heavy lubricant. If the chamber was oversize, it served as a fluid fit to make up the deficiency and, if unsafe head space existed that would result in case rupture if ammunition was fired dry, then the lubricant allowed the cartridge case to slip back at the start of pressure build up, to take up the slack between the breech lock and the breech lock key. Had this method of "quick fix" not been possible, the Navy would have long ago recognized the seriousness of the situation. In fact, this inexcusable method of correction was in use so long that it was becoming accepted as a satisfactory solution of a necessary nuisance.
The Oerlikon 20mm machine cannon was used on basically everything during WW2. It was the most common machine cannon made during WW2. It and its greased ammunition was used on American and British fighter aircraft, anti aircraft installations on ground and sea. PT boats had the things bolted to the deck.
The grease had to be applied by hand as the ammunition was linked. This was labor intensive and messy. You can see at exactly 2:14 on this WW2 video a Sailor’s hand painting grease on the 20 mm ammunition loading machine for the Oerlikon anti aircraft machine guns.
http://www.youtube.com/watch_popup?v=9dR3h2HdnBQ
After WW2 it was decided to get rid of the grease and the Oerlikons used through Vietnam by Patrol boats had oilers installed. There were numerous tests conducted which Teflon coated cases were tried, and chamber flutes, but the cheapest and simplest solution was to install oilers.
If you can control the total press/linkage flex as shown above coupled with a finite length of brass sizing every time you can control that case stretch much more closely and have longer case life. I would not throw away those cases you sized to min specs but instead reload them (or add a few more sized the same way so you have a good sized sample to reload together) and see what the case life difference is. You will have to keep them isolated from the rest though to do this. One sizing like this will not turn the brass into little bombs IMHO, but just shorten the case life a bit. Just use them in the bolt gun. You should always check your bottle necked brass for the internal rut every time as part of your brass inspection. I always do and have been safely reloading without case head separations for many years.YMMV
