Starting a STEN Mk V SBR in the US

[tiriaq]

Could you use it with a 16 1/4" barrel in the meantime, or would that muddy the waters?

[17thairborne]

No, that would be perfectly legal as far as I know, I just did not want to spend the dough for something I would not use more than once. The only problem I can foresee aside from the $ would be the tuning of the weapon. The process for tuning the gun involves firing single rounds and evaluating if the hammer and breech block springs need to be shortened or not. I would think the longer barrel would generate some greater blow back forces (could be wrong here) and I would have to have two sets of springs, one for the 16" barrel and one for the shorter one.

DOes that sound right?

[tiriaq]

Very well could be. I have fired the same carbine with both original length and long barrels, and it certainly seemed snappier with the long barrel.

[17thairborne]

That would certainly get me on the range much earlier than waiting for the ATF!

[Vincent]

DOes that sound right?

In a way, yes and no.

We're dealing with Newton's third law here. So it's mostly about mass (bolt weight). The longer the barrel, the more mass needed. The force pushing the bolt back hasn't increased. Only the time the bullet is in the barrel has gone up.

Without more mass the casing will be further back in the chamber when the bullet exits the barrel and the pressure drops. Not an issue for your Indy Ord bolt. It has plenty of mass. But with light bolts that relied heavily on advanced primer ignition as full-auto's, the casing can travel too far back in the chamber before pressure drops causing the case to bulge, split or rupture. The PPSh41 and PPS43 are good examples. With long barrels and lighter semi-auto bolts they will split a lot of cases and have very dirty breeches. But who reloads 7.62x25mm?

A stronger recoil spring will have a higher initial closing force, but there comes a point where the weapon becomes hard to charge. That can be overcome to a degree with a double spring system, like the Sterling has. The light spring makes charging easy, while the stronger spring captures most of the energy.

There's an section on blowback gun design in "The Machine Gun" by George M. Chinn. I think it's in volume 4. It also has all the formulas and it's a free download.

For my semi-auto Sten I made a small spring cup. (Same concept as the Sterling spring system). The cup keeps the light spring from reaching its solid length (fully compressed) and getting damaged when the bolt cycles. The cup has a hole in its center and slips over the spring guide rod with about 1 3/4" section of stronger spring behind it.

Thank you for documenting your build here. It's a lot of effort and much appreciated.

BTW, Building is very addictive.

[Peter Laidler]

Thast's a well thought out piece Vince. You know my views on API of course but that doesn't affect this situation of barrel length here. The Beretta 30's type SMGs have a long 9mm barrels and while it's not comparing like with like here, it's worth pointing out that we regularly used to fire them with standard government Mk2z ammo with no ill effect or signs of bulging.

What is interesting is that in our SMG barrels, they never exceeded the optimum 7" length. Which leads on to another question........... Who decreed that 7" was the optimum length? And optimum length for what? It COULD be the optimum length for mediocrity! Another interesting point was that during manufacture, breech blocks were constantly weighed and before being passed. had to weigh-in within a set of tight criteria. This was important when a different steel, of lighter or heavier density was being used and the weights were controlled by the depth of the central boreing in the rear end.

Oh for a lab where we could all gather after school for some extra after school physics lessons............. You mention Newton...... I used to take a Sten gun (dewat of course) into those lessons knowing that it was one thing that would really hold attention and make it a memorable lesson for its relevance. I used to wrack my brains for a similar small arms related lesson that could be used to realistically illustrate leverage, forces anf fulcrums......

[Vincent]

Thanks, Peter.

I was wrong when I said, “the force pushing the bolt back hasn't increased.” If the muzzle velocity increases, and I think it does with a 16” barrel, that means the energy driving the bullet and the bolt increases too. Newton’s second law,,, F=m∙a or Force = mass x acceleration.

As for the optimum barrel length of a blowback open bolt 9mm SMG. It probably has more to do with the weight of the weapon than any of the other factors.

I am sure you have noticed how light Stens and Sterlings feel when they are stripped of all internal parts. The single heaviest part is the bolt (breech block).

If I remember correctly, there’s a significant increase in muzzle velocity between 7” and 10” barrels. Obviously this would necessitate a heavier bolt for the longer barrel.

There’s little increase in muzzle velocity with barrels longer than 10”, but they still require more and more bolt weight as they get longer. This is due to the bullet dwelling longer in the barrel. So, they are not what I would call “optimum” for a SMG.

At 7 ¾” the Mk4 Sterling barrel squeaks out a little more muzzle velocity and the bolt weight is still reasonable because of the double spring system and API.

Unfortunately we lose the API advantage when converting open bolt guns to closed bolt guns. So we need to add more bolt weight. The longer 16” barrel means even more bolt weight. Probably the best example of this is one you wrote about in The Guns of Dagenham, “Closing the Bolt.” The total recoiling mass of the Mk6 is considerably more than the Mk4. Not only is the bolt heavier, there’s also the heavy firing pin spring retainer included in the recoiling mass. It’s an ingenious conversion to meet US laws and all done before computers.

You reminded me of my boarding school math teacher. He had a training aid that held our attention for years. He was a large, barrel-chested retired Major General with a big bristling handlebar mustache and a booming deep voice. He had been a POW in Burma where the “Japs” as he disparagingly called them had removed his fingernails and two fingers from each hand. He would walk up and down the classroom with a riding crop tucked under his arm…when he wasn’t using it…on one of us. That was his teaching aid. Believe me, it worked!

[tiriaq]

Another aspect of bolt weight that I have not seen mentioned in this thread with respect to traditional submachine guns is the affect on cyclic rate. The lighter the bolt, the higher the cyclic rate.

In North America, it is simply not legally practical or possible to make a firearm identical to classic originals like the Sten. In Canada, there are privately owned original Stens, but ownership is limited to those who had them before a cut off date, and they cannot be taken to a range. '78 for autos, '95 for ones converted to semi. The IO, SAS and other similar operating systems allow a legal firearm to be made, using many genuine parts and retaining most of the appearance of the original. Shooting one of these reproduction Stens is not the same as shooting an original set on repetition. The sensation is different, because of the closed bolt function. The second pattern SAS uses a rather large cylindrical hammer; there is a bit of the sensation of movement that open bolt operation produces when the trigger is pressed, but the mass of the forward moving parts is much less.

[Peter Laidler]

Ah........ the light bolt theory Tiri...... This is one of those diminishing returns things where very soon the lighter weight does something - or rather DOESN'T do/allow something..... A bit like a lighter return spring thing. Let me read-up and come back on this. It is a well used discussion subject

When you've finished this project 17th, you ought to keep all this forum stuff in a file with your photos because you've certainly got some of the forum brain-power working overtime on your behalf here with some great thought provoking comments. Oh for that after school physics lesson. Just think of how we'd all be rattling through Newtons Laws and the laws of springs and compressions..... What is it now?

[17thairborne]

I never imagined this project would be so informative to me. Rather than a "wow...nice project", you guys are creating a classroom where free discussion rules the day.

These issues are truly providing me the beginning of an understanding of the dynamics of the many phases of the operation of the STEN. It reminds me of my early remembrances of firing, unlocking, extraction, ejection, cocking, loading etc. and having to memorize nomenclature;

"Sir, US Rifle 7.62 mm M-14 is light weight, air cooled, magazine fed, shoulder fired semi-automatic weapon"........

---------- Post added at 10:34 AM ---------- Previous post was at 10:27 AM ----------

Shooting one of these reproduction Stens is not the same as shooting an original set on repetition. The sensation is different, because of the closed bolt function. The second pattern SAS uses a rather large cylindrical hammer; there is a bit of the sensation of movement that open bolt operation produces when the trigger is pressed, but the mass of the forward moving parts is much less.

Can't wait to fire it. Wish I had the experience of firing a STEN on FA to make a personal comparison. I had an FA M3 at one time and the feeling of the open bolt firing system was much different from say an AR on FA. Obviously completely different weapons, but the bolt moving forward before firing was always a bit "weird" to me. Such a slow cyclic rate of fir though that permitted occasional single shot if the trigger was managed well. I used to go shoot with a few friends on the ABQ SWAT and they loved firing the M3.