While MkVIIIz is a popular topic...

Actions stretching? We had this discussion some time (and tome....) ago regarding L1A1's. How does a rifle action stretch? Put a bolt and nut between a slab of steel, tighten the nut up and the bolt will stretch - if you over tighten it! Because it has exceeded the limits of its mechanical elasticity. This action doesn't happen with a rifle as you have to stretch something between two (or several) points. The bullet passes through the bore and the rifle recoils. The bolt is under tremendous compression and the body might measurably twist and warp and then resume its shape. But it isn't subject to tension.

Many things happen to rifle bodies but 'stretching' ain't one of them!

Well there's no arguing with you. It went out of headspace and no other bolt head would bring it back into headspace. So I guess you know what would have happened as I have no Idea.

The hardening in the locking lug recesses is probably worn out. That has nothing to do with stretching. The bolt will set back causing excessive headspace. I think it's mythology that the Mk.VIIIz cartridge has more pressure. The only difference is the 175 grain boat tail projectile as opposed to the 174 grain flat base projectile in the Mk.VII/VIIz.

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Originally Posted by Bruce_in_Oz

Mk8 was ONLY introduced to extend the useful range of said Bren and Vickers guns, (extra distance is GOOD on the two-way rifle range), NOT to supply "match-grade" ammo to the basic foot-sloggers.

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Exactly the case and it falls exactly in line with my experience and expertise in the theory of machine gun fire, it appears that ballistic experimentation nearing the later part of the First World War discovered that a BT bullet was more stable in the far ranges of direct and indirect MG fire. This is still practiced today with a variety of guns, mounts and sighting systems to ranges of 1,300 (approx tracer burnout) to +1,800m, the summary was that the FB bullet would deliver a beaten zone that was somewhat unpredictable, lacking density and the cone of fire was less than useful at extreme ranges. Whereas the BT bullet is more stable in flight at extreme ranges, not so much way to increase absolute range, but as a way to increase effectiveness of the weapons system at the beyond shoulder fired, direct fire distances.

At those ranges the MG would be used as an area weapon, usually in a grouping of several guns to deny ground and restrict movement, long bursts were used to saturate the ground with incoming projectiles.

In the most basic of terms a BT bullet at extreme ranges would deliver a smaller extreme spread of the "grouping" and deliver a more consistent beaten zone.

Sorry if I appear to have rattled you a bit Martin, but no offence meant at all. And I don't mind or feel threatened if you want to argue the point but mechanics and physics isn't on your side. I only came onto this (and ONLY this) site because I was asked to comment on something a few years ago. I say it as I find it and as it is from an Armourers perspective, and as an engineer to boot. There are many myths surrounding small arms especially from the old and the bold that have been passed down from generation to generation. Here's a few examples: The Sten will go off and kill everyone nearby if you drop it - it won't(?). The Bren is deadly accurate. It's not, it's crap. Rifles stretch........... I'm not alone here either. And the straightest talkers are the Australian contingent!

PS I was the last Vickers MMG trained Armourer left in the UK Military which gave the UK MoD an opening to use our remaining guns

Hi Peter, I'm not rattled but there is no point in arguing about it because I know you know your stuff. I had the same thing hapen to a SMLE in 303-25 that I loaded too hot. But as I said I have no Idea thats why I am here.
Cheers.

Bodies stretch, alright. In the "good old days", Oz rifle club armourers had spare ones just for such occasions.

A classic example is the fate of the Lithgow experimental 7.62 NATO carbines built on SMLE actions in the mid-1950s.

The remains of those I have seen are DEFINITELY "distorted". The right-side rail, being the thinnest piece in cross-section was visibly stretched and the body effectively bent. HOWEVER, NONE were reported to have "failed catastrophically", i.e. (blown up). Bodging the mag well to accommodate an FAL mag probably didn't help, either.

The poor old things were originally made from a simple Carbon-Steel alloy, as per the original early 20th Century recipe, and as opposed to the No4 which employed a different brew.

The locking shoulder / recess were "spot-hardened" using pointy carbon electrodes to raise the temperature followed by a quench etc. If you look closely at a near-mint Oz SMLE, you will see small, circular patches of discolouring on the exterior of the body, just behind the locking shoulders.

The use of oiled, proof-rounds for "stress testing" probably didn't help either.

These shoulders also WEAR quite rapidly once this surface-hardened skin gets too thin. Then you will also start to see "set-back" marks, mainly on the nice, big RHS one. By that time, your bolt body, if original, will probably be getting a bit tired too.