Explain the Original SMLE 303 Headspace Situaiton?

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Originally Posted by RC20

. . . I had one guy claimed the bolts compressed! Wow, really? . . .

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Yes, really. On firing, chamber pressure transmitted through the case head compresses the portion of the bolt between the bolt face and the locking lugs. Normally, when pressure abates, the bolt returns to its original dimensions. This is what we call an "elastic" process. If pressure is grossly excessive parts can be permanently deformed - a "plastic" process.

It may be helpful to think of what happens to a simple coil extension spring. Pull the ends away from each other less than the maximum design extension and it happily returns to its original length (elasticity at work). Extend it too far and it won't return to its original length (plastic deformation - or even fracture). Unlike the bolt and receiver of a Lee-Enfield during firing, this is readily seen by human eyes - but the same processes can indeed occur even though we can't ordinarily observe them with only our unaided senses.

Search for "elastic vs. plastic" and you can find all sorts of good stuff like this example -

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Elastic vs Plastic Deformation

Deformation is the action or process of deforming or distorting. When a force is applied to an object, the object will either compress or stretch as a response to the force. In mechanics, the force applied to a unit area is called stress. The extent of stretching or compressing (as a response to the stress) is called strain. Every material responds differently to stress. The response is highly dependent on the chemical bond type of the substance. Deformations can be elastic or plastic based on what happens after the stress is released. Elastic deformation is the deformation that disappears upon removal of the external forces causing the alteration and the stress associated with it. Plastic deformation is a permanent deformation or change in shape of a solid body without fracture under the action of a sustained force. The main difference between elastic deformation and plastic deformation is that elastic deformation is reversible whereas plastic deformation is irreversible.

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as far as bolt compression goes, and lock up issues. Look at the rear locking bolt on the lever action rifle. EVERY reloading source says a lever action cartridge HAS to be full length sized after each firing, due to the fact that the rear ward displacement of the entire bolt upon the rear locking lugs causes the cartridge case to stretch to far to allow it to rechamber safely/at all unless full length sized.

Using the SVT 40 analogy again, the Soviets concluded elastic deformation of the receiver (and other parts including bolt) could add around 0.008" to the measurable dimensional headspace for a given rifle. They were greatly interested in this because case ruptures were an issue during the early development of the rifle. This info is on pg 176 of the book by R. Chumak, unfortunately only available in Russian. In my own small collection of SVT 40's you can see the progressive addition of meat to the receiver sidewalls from 1940 to 1944, presumably to stiffen it up.

Ridolpho

I wonder if someone could resurrect an old thread that I wrote a couple(?) of years ago relating to the mysterious stretching of rifle bodies and.......... I really don't want to go through it all again but it does ask some valid questions relating to what exactly are you stretching it against pray tell.
Peter Laidler.

This has taken a turn, so I appreciate the responses and will go over to the disagreement part.

You can't have steel compress. Its possible a receiver can stretch and spring back. I don't think so but put that in maybe.

If on firing you have increase head space, you may be moving the lugs to full tight, the bolt may compress if it has a floating head in that the spaces get taken up (this assume a rear locking lugs, a front locking floater will not do that)

But if it opens up and stays opened up, then its plastic deformation of the receiver and it will continue to grow on each shot. Once you hit plastic it does not stop (assuming save force that put it there)

A cap screw (fastener) has to have threads cut into it to be a spring. A rifle had no parts in it in the receiver or the bolt remotely like that.

An extreme example is pipe testing. You don't test with AIR. Why? Its compressible and will continue to expand on release (messy if not deadly)

You pump the pipe full of WATER (which is partially non compressible) and then you top it up with air. Pipe ruptures, small release, no spread, no death.

A rifle bolt MIGHT flex, but its not going to compress.