Haganah - TMT IV Sten markings

Armourers and the designers will say that there really is no such thing as API. All API is, is simply a coincidental by-product of a breech block having a fixed firing pin set within a recessed bolt-head. Nothing more or less! And you MUST have a recessed bolt head otherwise the base of the round would foul the protruding striker as it was sliding across the breech face while being centred and fed into the chamber. I wrote a long article detailing the technicalities of this for the forum several years ago.

Blow-back is a more accurate description I feel but others say that the operation of blow-back is really just another form of piston operation. The spent case operates as a piston for several milliseconds

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Originally Posted by Peter Laidler

Blow-back is a more accurate description

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I agree, I remember the article and that was the one I referred to...was it really SEVERAL years? How time flies. I always liked the description of the '21/'27/'28 Thompson..."Retarded" blowback...because of the Blish principle. Pretty funny, reading that when I was about 12...

I sent the Americal Thompson Owners people copies from the late 20's/early 30's Ordnance Board proceedings relating to trials and tests on the Thompson. The experimental lab at Enfield decided that the blish lock (and the cutts compensator incidentally) was a frill if not a direct fraud!. It was nothing to with the coefficient of friction between the bronze and steel/dissimilar metals that caused the delay but just plain old friction PLUS the time it took for the H block to slide up, align and then commence to move rearwards. I seem to recall that they also argued against the notion that the initial movement of the breech block rearwards as the H block slid upwards slightly allowed primary extraction. This was on the basis that there's no requirement for P/E with a parallel case. Enfield also stated that the gun would work without the lock.

Mind you, the whole report was worded in a 'sour grapes' sort of way.

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Originally Posted by Peter Laidler

Enfield also stated that the gun would work without the lock.

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That's an interesting statement but it would mean that they made some thing to fit in between the bolt and actuator or you can't pull the bolt back to cock the gun. There were trials for the desert in which the bolt and actuator were mated by some handy armorer to eliminate the extra friction of the three parts with grit and sand present. I have no proof or pics of this trial. Just hearsay. It makes sense though, as I had one for many years and examined all these little things in detail... The Cutts may not have done much and simply directed the burned powder in front of your eyesight, but they sure look cool. A b*tch to clean sometimes too.

"Compensators" on 1911-type pistols in .45ACP are a bit of a waste of time too.

Mil-Spec ball runs at quite low pressure for a cartridge that size.

"Stiff" 9mm para, .38 Super, 9 x 23 etc are a whole different thing, especially the last two running +P loads.

LOTS of HIGH-pressure gas available at the muzzle to be deflected upwards and even rearwards to tame muzzle rise and speed of the action opening.

Given the "loaded" weight of a '21 Thompson, recoil and muzzle rise are not major issues.

The other tricky thing is "headspace".

Old John Moses designed the 1911 pistol with a chamber that effectively has NO "stepped" form at the front of the chamber. This "step" between chamber proper and the bullet leade seems to have been "optional" in several 9mm items as well.

The honking big extractor on the 1911 holds the case "close" to the breech face. The striker is "floating" and is fitted with a return spring. When smacked by the hammer, it is capable of protruding a bit more than needed and the floating striker, with its return spring, is able to ignite pretty much any cartridge held by the extractor and set things in motion. The system has worked OK for a few years now.

.38 Super is SUPPOSED to headspace on the minuscule rim. MMMmmm: the fact that one can, at a (quite expensive) pinch, run several of the "other" European 9mm rounds through a .38 Super chamber raises some questions.

The modern "classic" is .357 SiG. For those not familiar with this beast, it is essentially a .40 S&W necked down to use .355 / .356 bullets and performing sort of, like a short, fat, .357 Magnum. Based on experience with rimless rifle rounds, one would expect it to headspace on the shoulder, but, no; it is designed to "headspace" on the case mouth.

I would imagine that the point the DDE(trials) at Enfield were making (see BAR thread 34) was that even if you snipped the extending lugs off the H block, the original Thompson gun would still operate satisfactorily in a true blow-back mode. They must have produced a steel H block in order to ascertain that the delay was pure friction as opposed to increased friction between dissimilar materials. It even described the method, results and conclusions of the trial. With a bit of thought and adjustment, this is what the later M1A1 turned out to replicate.

Bruce has good points, specially about muzzle breaks on low pressure cartridges. It need to be something very snappy before they do anything. I used 9mm para in a Star "B" in Largo and it worked when fired from a loaded mag. The extractor did the job too.

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Originally Posted by Peter Laidler

snipped the extending lugs off the H block, the original Thompson gun would still operate satisfactorily in a true blow-back mode.

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Yes, Peter, I'd think they just made something to hold the two together, even just take the side lugs off and then it worked like any other.

I wasn't aware that the U.K. produced cover was welded at the rear, Vincent, and I don't recall noticing any welds when I've had a cover off. I have several examples of the Sten which I can look at and so I will look in detail at the cover. If they are all welded as you say at the rear it must have been found that the metal wouldn't form into the required shape without it i.e. it cracked/split/buckled. It's possible that the profile of the blank included a couple of slits which were then welded up after forming. I also have a Canadian made Sten and would expect that cover to be produced exactly the same way as the U.K. cover.

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Originally Posted by browningautorifle

That's an interesting statement but it would mean that they made some thing to fit in between the bolt and actuator or you can't pull the bolt back to cock the gun. There were trials for the desert in which the bolt and actuator were mated by some handy armorer to eliminate the extra friction of the three parts with grit and sand present. I have no proof or pics of this trial. Just hearsay. It makes sense though, as I had one for many years and examined all these little things in detail... The Cutts may not have done much and simply directed the burned powder in front of your eyesight, but they sure look cool. A b*tch to clean sometimes too.

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Jim, I seem to recall. & I THINK it MIGHT have been in the Thompson Book. From the Publishers Who also Incidently. ALSO Publish superb works from a 'Well Known Author' on this Forum.....;)

That there was a Mod where the blish 'H' pieces were removed. & a Nut & Bolt was fitted transversely across the Bolt in the recess. Where the Blish lock sat. This gave a 'Bar' like facility, for the underside of the cocking slide. to engage & enable the bolt to be pulled to the rear. The gun then was true 'Blow back' & not 'Delayed'.

Regarding the Sten trigger mech cover. There were loads of manufacturers of this piece and there were at least 3 variants of it. Screw holes, locating dimples and screw-holes AND dimples plus the various manufacturers types. Manufacturers were given the spec and manufacturing detail. But they were allowed to make representations to the MoS for relaxations to parts of this in order to make the best use of their facilities, material in hand and costs. So there were fully pressed covers, folded and welded as shown in the Sten book, punched, folded with ears spot welded etc etc.

You could write another book on just the covers if you were a real rivet counter type. Indeed, I relate the story in the book of how one small company in Wiltshire punched them out by the thousands