Lithgow SMLE Failed Field Headspace Gauge

[Bruce_in_Oz]

Apart from "Match" M-1s and M-14s, I suspect not.

How many "interesting" Mausers must there be out there? How many Mauser owners have even seen a headspace gauge for their rifles?

It is probably to the good that many Lee Enfield enthusiasts are aware of the issue; the problem of course is, what to do about it.

We have all heard tales of No1 actions "stretching", especially from the old full-bore shooters. Given the fact that the action was not supposed to stretch in any meaningful (measurable) way during the various proofing stages, it is unlikely that the thing would stretch much from the use of Mk7 ball. The catch here is that the metallurgy of the SMLE was nowhere as advanced as the No4 and the techniques for achieving the various structural and hardness states were just plain archaic. Thus, the receivers were slightly "over-engineered" for the cartridge. Note again: the rifle was built around a specific cartridge, not like sporters, where the rifle is built to accommodate a huge range of possible ammunition, plus some "creative" ammunition variables.

As our Indian cousins clearly showed, the SMLE is quite capable of consuming a steady diet of "hotter" ammo than Mk7 .303; i.e. 7.62 NATO. Post WW2 advances in alloying and heat-treatment techniques and instrumentation helped a lot.

In a sporter, headspace is important for ignition reliability and brass life. Ordnance and armourer types seem to not worry about brass life, but are pretty keen of consistent ignition and achieving specified accuracy. For a given cartridge, correct headspace will enable correct ignition and thus reduced deviation in bullet velocity and thus group size. Light striking, either from excessive headspace or a dodgy spring will induce "micro-hangfires".

The thing about Mausers and Lee Enfields is that when fed from the magazine, as nature intended, when a cartridge rides up during feeding, the rim slides up the bolt face and behind the extractor. Relying on the extractor to hold the case firmly enough to allow good ignition in a case of excessive headspace is pure optimism. It might go bang, but it might not be the "proper" bang.

Thereby hangs the tale of why military primers are generally crimped in. It reduces the tendency to "back out" during ignition. One of the classic signs of dodgy headspace is primer "back-out". In any sort of case this has ONE cause: Excessive headspace. NOTE: this excess can be caused either by defective ammo or a defective rifle, or both. If you push the shoulder of a rimless case back beyond specified datum, you have just induced a headpace problem. If the rim on a rimmed case is too thin: ditto. If the relative locations of the datum surfaces on the chamber and barrel are further apart than specified, again, excessive headspace.

It gets better with rimless cases. If you get a bunch of perfect" cases for your nice Mauser, correctly seat a primer (and only a primer) and then fire these primed cases in old Helga, you will note that the primer has partly backed out of the pocket. This is because the detonation of the priming compound has blown the case FORWARD. This action has the interesting effect of using the chamber as a sizing die and pushing the shoulder BACKWARDS. Thus, CARTRIDGE headspace is no longer correct. "Squib" loads will do the same thing. In a real cartridge, this also happens initially, BUT, when the main powder charge ignites, the case expands in all directions and is also blown back against the bolt face, re-seating the primer. If headspace is excessive, the lateral expansion of the case, especially at the neck region, will make it grip the chamber walls before the rest of the case is blown back towards the bolt. Hence the primer "hangs out" of the pocket and nasty stretch regions appear towards the base of the case. A couple of cycles of resizing and firing and the brass WILL crack at those stress points from pressure applied to this work-hardened area.

The same thing happens to rimmed brass like the .303 except there you can also have a chamber that is cut too deeply. In this case, (boom! Tish!) the shoulder gets shuffled back and forth during firing and reloading AND, if headspace is excessive, the primer shuffling and work- hardening happen at the back end as well. In the mud of Flanders or the jungles of Malaya, this was of little concern to anybody, however...............

And then there is the radial stretching of "oversized" chambers.


Be prepared to write off brass sooner rather than later in a "mil-spec" .303 chamber.