Lanchester barrel question

[ProLibertate]

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?

Information

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[bigduke6]

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

[Peter Laidler]

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

[browningautorifle]

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

[Archilochus]

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.

[bigduke6]

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.

[Peter Laidler]

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?)

[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? (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.

[Archilochus]

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?

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.

[browningautorifle]

The flat is at the bottom on the Sterling BAR.

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...