Bolt thrust and chamber lubrication in a friend’s 6 BR

[BoydAllen]

Some time ago a friend told me of a problem that he was having with a highly customized 6 BR. It seems that every time that he cleaned the barrel, fired cases from the first two shots showed classic signs of an over pressure load. After that, with the same load, the signs went away. Without going into a long story, it turned out that his habit of “drying” the chamber,after cleaning the barrel) with a chamber swab, that would be reused for that task numerous times was the problem,the swab retained some oil after each use).

Getting back to the brass, the fired cases from those first shots showed abnormally flattened primer corners, and a bright spot where the brass had been force into the ejector hole in the bolt face. Cases from subsequent firings did not show these features. When he began using a fresh .30 cal. patch,wrapped around a swab) and alcohol to dry his chamber, the problem went away. It has hot reoccurred since. Before, it happened consistently.

I might add that I think that the problem arose to such a noticeable degree, in this case, because although the 6BR case has just as much head area as a 30-06, it has much less body length with which to grip the chamber walls, and because his gunsmith,,of national reputation), likes polished chambers.

I believe that it was sometime after that, that Varminnt Al, for whom I have tremendous respect, studied the subject and came to the conclusion that lubrication did not have an important effect on bolt thrust. It was a very elaborate piece of work, BUT it was not with actual cases in chambers. Because of that, I think that my friend’s actual experience trumps his study.,Yes sir, I know that you said that the boat couldn’t sink, but I am treading water while talking to you on my cell phone. ;-)

 

[NorCalMikie]

Well said Boyd! :thumb: I may have run into the same problem and not realized it. Dry chamber from now on.,and brass too)
All my brass gets a high polish and I can see where "that" might contribute to less friction. My brass will still "shine" but the chamber will be dry!

 

[jo191145]

Boyd
In the shooting world I am just an annoying fly speck on the target.

Nonetheless The only difference I can see between bolt thrust from lubed to non lubed or polished /non polished chambers would be the amount of force consumed in stretching the brass.

Either way the bolt is gonna get whacked. The percentage of energy required to stretch the brass is miniscule I would think.

Now I could believe if one were to have one of those super tight necked chambers and some high viscosity,read thick) oils or sixing wax got trapped in there during firing that could raise pressures.

Looking for further education.

 

[BoydAllen]

I was going to address this in a later post but to your point of how much pressure it takes to stretch brass to the bolt face, let me give you the following example.

Many years ago I had a Springfield that had a newly installed Smith Corona 03A3 barrel. One day I decided to do a little experiment. I went to the range with all the necessary tools to reload 30-06 with a Lee Loader,neck sizes only, no expander), including an old Hollywood micrometer type powder measure for which I had made a setting table for AA2520. I decided to work with some 150 grain bullets that I had at the time, and noted the loads for that weight from a manual, deciding to start with a middle of the book load.

It is at this point that I should mention that the barrel interior was a bit on the rough side, but with no function problems. I think that the chamber must have been as reamed,no polishing) and it may have been that the bore was not plugged when the Parkerizing was done.

In any case, I made up some loads at home, perhaps half a dozen, and when I shot them, I noticed that the primers were slightly protruding. Curious, I reloaded and fired the brass several more times with the same load. Each time the brass came out with the primer backed a little farther out of the case. The last time, even though the brass OAL was correct, the shoulder had been moved so far back on the case that the neck was starting to be reduced in diameter at its end by the angle at the end of the neck portion of the chamber. After that, being young and impetuous, I decided to risk a full pressure load, and after that the cases came out with the primers flush and a bright line just above the case heads, denoting the presence of what is called an incipient separation. At that point I flattened the necks to prevent the reuse of the cases, and threw the cases in the trash.

This is what had happened. The Springfield has a very heavy striker assembly, and that particular one had been fitted with the heaviest spring that strong hands could manhandle onto it. The '06 has a relatively small shoulder angle, and the heavy whack from the weighty pin was driving it forward in the chamber. At that point the pressure generated by the burning powder became great enough so that it was held in the forward position, so that the only way that the head could have come back to the bolt face would have been for the brass to stretch just in front of the head, which in the case of the first firings, did not happen.

Now to your point. Middle of the book, jacketed bullet loads' pressures are not miniscule by any means, yet in this case they were insufficient to stretch the case back to the bolt face.

Now imagine a rifle chambered in a caliber of the largest available head diameter, one that nears the safe design limit for a standard bolt, designed at a time before the cartridge was available. Now consider that the owner of this rifle is new to reloading, and being a red-blooded American male, wants to be able to tell his friends about how much velocity his new rifle produces. Looking for tips, he goes on line and reads that lubricating his cases for their first firing is a good thing, so he loads up his slightly bolt sticky load, in new lubricated brass. Let us say that he has also decided to begin investigating the advantages of seating bullets into the lands, but unfortunately the post he read failed to caution him to drop his load back and work up with the new seating depth. Can you see how the various factors could stack up to an undesirable conclusion?

Experienced loaders may on occasion lubricate cases for a particular application within narrowly defined limits. Recommending that this be done to a world audience of unknown experience level, is another thing. The fellow that is reading the post may not have enough background to know when it should not be done. Long and short, some things are better left to more private discussions where the speaker,or writer) has more information about the experience level of the reader.