Rob, that's interesting, as it's for the 75mm. bored out 6pdr. barrels that were used on some british tanks, later in the war, the chamber was the same as US 75mm. Sherman tank, M18 as you say, as that ammunition was used for these.
I had a barrel with schrapnel damage years ago, externally same as 6pdr. interrnally 75mm.
Cheers,
Lex
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