Lanchester barrel question

I'm building up a semi-auto Lanchester from original, Indianapolis Ordnance and hand built parts. I've never held a Lanchester barrel in my hand; I've only seen pictures. As I live in CA, building an SBR is entirely out of the question for me (as is moving out of this state for a few more years) and I'm reluctant to buy an original barrel and get charged with constructive possession.

So.... I'm building my own 16"+ barrel based on a gunsmith blank from Green Mountain Rifle Barrel company.

In looking at all the pictures of original barrels that I can find, on the chamber end, eject side, there is a flat machined. I've been looking at my receiver tube and semi-auto bolt assembly. I juts don't see why there is a flat. There is no feature in the tube that needs be cleared and no feature on my bolt that needs that space either.

Why is the flat there? What am I missing?

https://www.milsurps.com/images/impo...05325299-1.jpg

Only reason for the machined recess is for the clearance of the extractor claw?

Anyone from the Sterling Lanchester clan want a quick history lesson? I don't really know what that flat is for either but I do know that when Sterling started to make the Patchett Sterling guns, they started off with 6 thousand or so part finished Lanchester barrels in various stages. So when the opportunity arose, they completed them by rotating that flat around to the bottom of the barrel. They claimed that the reason for this was that any sand or whatever would get pushed forwards by the breech block and be pushed UNDER this flat and out of harms way into the void of the front barrel casing. Have a look at a Sterling barrel and you'll see.

Alas, the reality didn't quite match the hype........... Because while sand, grit and other debris might get pushed forwards, it just fell into the far LARGER oval opening behind this little gap - right into the trigger mechanism! But it stayed the course and the legend lived on.

This little gap also caused a bit of a problem when Patchett (and a major part by Bert LIST don't forget...) were formulating the Mk5/L34. High pressure gas venting from the ported barrel was leeching through this gap and into the mechanism. Something you definitely DON'T want!. So they solved this by re-designing the rear barrel stop/support to accommodate a small locator. Now, the Mk5/L34 barrel flat seats/locates on this locator flat which also seals the gap. All a reminder of its Lanchester ancestry.

Another bit of useless Lanchester and Sterling history

Peter, doesn't that flat spot sit on the right hand side of the gun, at the 3 o'clock position?

On the barrel I made for my Lanchester semi-auto (posted in an earlier thread here), I just omitted that flat, since I could not figure out any purpose for it. Works fine. I used a STEN-type extractor - maybe there's some difference with the original Lanchester type that requires the flat be there?

BTW - It's probably not "legally" required, but to keep 'the man' happy, I'd suggest making your barrel threads different than the original, so that an original short barrel can not be dropped in. Or maybe that IO tube is reduced diameter, which would take care of that.

Well you learn something everyday........I thought it was due to being a direct copy of the MP28, in which the extractor stands slightly proud of the bolt.

The flat is at the bottom on the Sterling BAR. The Lanchester extractor is anchored about 1.5" rearwards of the extractor claw and is retained by being pressed in a hole. Over engineered and prone to breaking. A modified extractor was made with better steel and heat treatment and marked M on the shank. Academic here but it doesn't stand proud of the bolt (well, it might be a few .001"s or so but the claw end was shaped to fit into that wide slot. Don't tell anyone I told you, but just use a Sterling Mk4 breech block, reposition the cocking handle by grinding away the .014" deep case hardening and drilling a new hole - and be done with it!

We used to rebuild loads of these for the Aust, NZ and Royal Navy that came in from the big NoD at Sembawang. They were part of the coastal patrol boats and submarines and were usually minging. But once they'd gone through the system they were as good as gold. Good fun range testing them because we were only meant to shoot short bursts. But how do you just shoot short bursts with an auto fire rock and roll Lanchester. It's very difficult when you're all just 20 years old and the ammo is free. No shortage of most spares as I recall and there were still 6 or so in the workshop armoury when we pulled out. Left behind from an ex UK submarine that was given to Australia (HMS Aurochs?)

Thank you everyone! I'll be following in Archilochus' footsteps and omit the flat.

From what I can see of the mechanism, the length of the barrel isn't a really critical dimension. It looks like it could tolerate even as much as +-0.125" Yes? (aside from legality of course). I can see a faint wear mark where the original barrel rested and I'm going to make mine reach to roughly the same mark.

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Originally Posted by ProLibertate

Thank you everyone! I'll be following in Archilochus' footsteps and omit the flat.

From what I can see of the mechanism, the length of the barrel isn't a really critical dimension. It looks like it could tolerate even as much as +-0.125" Yes?

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Since it shares magazines with the STEN, I located my barrel similar to the STEN, with the breech end of the barrel about 0.040" forward of the inside front face of the magazine opening. I've seen STEN Mk3 kits where this measurement was zero, up to almost 1/8", so I went somewhere in the middle.
Once the rear end of the barrel was located, I just made the front end (not including the extension) flush with the front of the tube.

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Originally Posted by Peter Laidler

The flat is at the bottom on the Sterling BAR.

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Yes...but this barrel for the Lanchester I mean. The mag is on the left and so is the feedway, I wondered if the flat shouldn't be on the right...

I really don't know why the flat is on the right on the Lanchester BAR. I would just assume that it is because that's how it was on the MP28-II. Why was it on the 28-II? I can only assume that because the 28-II extractor extended to the full diameter of the tube (can anyone put a photo of one up?) and protruded out of the front of the breech block the flat would allow any rubbish to clear. Otherwise it'd prevent the BB from going forward fully

A good modification to a standard Lanchester is to insert a grub screw into the side of the BB top hold the floating firing pin assembly fully forwards. This eliminates the original inertia firing pin. Not as good as a Sterling breech block though. The even better news is that you can still use the Sterling BB in a Sterling!

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Originally Posted by Peter Laidler

Why was it on the 28-II? I can only assume that because the 28-II extractor extended to the full diameter of the tube (can anyone put a photo of one up?)

A good modification to a standard Lanchester is to insert a grub screw into the side of the BB top hold the floating firing pin assembly fully forwards. This eliminates the original inertia firing pin. Not as good as a Sterling breech block though. The even better news is that you can still use the Sterling BB in a Sterling!

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Here's an image of the '28 BB with extractor ...
https://www.milsurps.com/images/impo.../gXzBWaj-1.jpg

That's a fine idea for modifying the firing pin on a full-auto Lanchester. Unfortunately most of us in the US can only fool around with semi-auto, closed-bolt guns without the fixed firing pin.

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Originally Posted by Peter Laidler

I would just assume that it is because that's how it was on the MP28-II.

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Lots of stuff was that way, just because...

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Originally Posted by ProLibertate

In looking at all the pictures of original barrels that I can find, on the chamber end, eject side, there is a flat machined. I've been looking at my receiver tube and semi-auto bolt assembly. I juts don't see why there is a flat. There is no feature in the tube that needs be cleared and no feature on my bolt that needs that space either.

Why is the flat there? What am I missing?

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There is a feature inside the tube that needs to be cleared. It’s the front sight.

The Lanchester front sight sits in a dovetail that’s cut into the tube. The bottom of the front sight is flat. The flat on the barrel allows it to clear the bottom of the front sight.

Attachment 93743

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Originally Posted by Vincent

There is a feature inside the tube that needs to be cleared. It’s the front sight.

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Well dang - I went and installed my front sight after the barrel. And now I learn what that flat is for!

Thanks for the information. Off to my shop to drive out the front sight and cut that flat.

Quote:

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Originally Posted by Peter Laidler

This little gap also caused a bit of a problem when Patchett (and a major part by Bert LIST don't forget...) were formulating the Mk5/L34. High pressure gas venting from the ported barrel was leeching through this gap and into the mechanism. Something you definitely DON'T want!. So they solved this by re-designing the rear barrel stop/support to accommodate a small locator. Now, the Mk5/L34 barrel flat seats/locates on this locator flat which also seals the gap. All a reminder of its Lanchester ancestry.

Another bit of useless Lanchester and Sterling history

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Interesting, Peter. I hadn’t thought about gas venting through there.

When they designed the closed-bolt semi-auto guns, they made the blocking bar that prevents a full-auto bolt from being used just low enough to allow the flat on the Mk4 barrel to clear. Do you know why they did that? I don’t think it’s a coincidence.

Looks like 0.059” from the inside of mag well to barrel breech face.

That's 1.5mm. So, probably taken directly from the MP28.

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Originally Posted by Vincent

There is a feature inside the tube that needs to be cleared. It’s the front sight. The Lanchester front sight sits in a dovetail that’s cut into the tube. The bottom of the front sight is flat. The flat on the barrel allows it to clear the bottom of the front sight.
Attachment 93743

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Just a bit of an update. I had left the flat off on my build, and have now found that the flat is not needed if the receiver tube is of a reduced inside diameter. The added wall thickness means the bottom of the sight does not protrude into the tube.

How you gonna modify the trigger?

The original Lanchester had a modified MP28 fire control mechanism. The tripping lever has a hinged extension that trips the trigger bar for semi-auto fire.
Some pictures here MP28 II Torch Cut Receiver Tube Section with Fire Control Parts, 9mm, *Good* - MP28 II Schmeisser - More - Submachine Guns - Machine Guns

Quote:

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Originally Posted by Vincent

The original Lanchester had a modified MP28 fire control mechanism. The tripping lever has a hinged extension that trips the trigger bar for semi-auto fire.
Some pictures here MP28 II Torch Cut Receiver Tube Section with Fire Control Parts, 9mm, *Good* - MP28 II Schmeisser - More - Submachine Guns - Machine Guns

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Them parts fit in the lanchester trigger group? Trigger only thing I’m stumped on my build.

You can modify the tripping lever to work. That and a spring is all you need. Or just make a tripping lever. You don’t want the hinge if it’s going to be semi only.

Does your trigger bar have the hinge welded?

Quote:

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Originally Posted by Vincent

You can modify the tripping lever to work. That and a spring is all you need. Or just make a tripping lever. You don’t want the hinge if it’s going to be semi only.

Does your trigger bar have the hinge welded?

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I got the mki*. Basically three parts. Trigger, a rod with a hole in it and a sear.

Regarding semi-auto firing mechanisms, have a look at how IMI did it decades ago, here: https://www.uzitalk.com/reference/li...l%20manual.pdf

Vaguely reminiscent of the system in a VZ58.

Cheers.