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?

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