How could they not catch this twice?

[crazy4milsurps]

So I bought a No.4 Mk1 that was packed with grease from importation and after I clean the bore, I see this(pictured) how is it possible that this passed the build process and at least 1 FTR without being caught? barrel is original serials to the receiver and what lucky B'stards fired this without blowing up?

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