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Old March 21, 2013, 10:27 AM   #1
Slamfire
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Join Date: May 27, 2007
Posts: 5,261
FireForming Brass and oil lubricity

I was reading the May 2013 article “Fireforming Brass” by John Barsness from Guns Magazine, and I am a bit confused by this statement:



If the headspace is a little long but cases will still fire consistently, oiling the case will allow it to slip rearward without stretching. No, this doesn’t increase bolt-thrust. Any suitable oil loses its lubricity at pressures below 10,000 psi. Once pressure rises to 10,000 psi, the case grips the chamber wall firm.

https://gunsmagazine.com/fireforming-brass/

What I am confused about is the “loses its lubricity at pressures below 10,000 psia”, I think he means above 10,000 psia, but if it is above, what happens between the case and chamber above 10,000 psia? Does the case lock in? I have been looking on the web for information to find if at high pressures “oil loses its lubricity” but I have not found anything to verify this. Anyone know?


I did find this historical account from General Thompson, the inventor of the “Tommy Gun” . This sort of supports the idea that something happens at high pressure with oil. General Thompson's early designs must have relied on oil for the “Blish effect” to work. His later design used dissimilar materials: the bronze locking wedge in a carbon steel incline, where the wedge was fixed in position at high pressure. I have attached pictures of this incase someone has never seen this locking mechanism.

From Arms and the Man 1 Nov 1920

“John Thompson’s surprise Party” by Capt E. C. Crossman

General John Thompson:

So the submachine gun was fitted out with felt oil pads in the rear of the receiver housing, over which or by which the bolt slides at each stroke. The “gun crew” has nothing to do with the oiling beyond squirting some more into the pads every 500 rounds or so. …

The locking bolt is the working out of a theory which I have followed for some years with much interest. Some years ago, trying to blow up a Winchester Model 1886, one of the best and most popular of the lever action type, I found much to my surprise that the bolt would unlock, and would back out of the length of the shell in firing. The conditions permitting this, were, the removal of the lever catch on the tang, and second the oiling of the cse, combined with an action pretty old and worn smooth and free of extra friction.

The gun did not blow open, it did not open under heavy pressure, it was entirely safe so far as that is concerned. It unlocked when the pressure was nearly off, and the bolt slid gently open, taking the fired case with it.

I wrote up this peculiarity, and a Navy Officer, Commander Blish, called on me, and discussed the matter. It developed that he had found by experimental work, combined to a few cases of inexplicable opening of the breech bolts of big guns, firing blank cartridges that any arm which has a bolt cammed entirely shut, as by a locking bolt of the type the M1886 referred to, is likely to cam itself open again when the breech pressure falls enough to “unlock” the friction surfaces the locking bolt in its grooves in the receiver and breech bolt.









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