Bramble
Here are the measurements of the bbl on my British double a 450 No2.

Just in front of the water table where the barrel becomes round. 1.19 [1.91 was a typo]
At 6" .915
At 12" .755
At 18" .677
At 24" .650
At 28" .660

The bbls are 28". I measured both bbls at 24 and 28 inches, the measurements were the same.

Hope this helps.